I think the biggest reason Rey/Finn are immediately liked and bought right away is because they're stereotypical caricatures that you've seen a million times already and have already bought into long before you've even seen the movie. The Mary Sue + Nervous goofball as archetypes are instantly memorable without further characterization.
Not that this is necessary a bad thing, but I don't think that ease/immediacy/recognizability alone really speaks to anything more about them being better characters.
Mmm...maybe subconsciously that helps but I have specific reasons for how I feel about each new character.
Rey: Despite being somewhat overpowered and lacking in flaws I feel there's a real honesty in Ridley's portrayal of this young girl so desperate to wait for her family and her belonging in the galaxy. That even when she is presented with opportunities to go on crazy adventures with the legendary Han Solo she remains committed to waiting for her family. She rejects her destiny in the beginning presumably becaue she doesn't feel it really is her destiny and that her future can only move forward when she reconciles with the family she's been waiting for since her childhood. She is wide eyed and amazed at all the new things she's seeing since leaving that desert ****hole of Jakku but it's not enough to deter her. When Rey finally heeds Maz's words that the belonging she seeks isn't on Jakku and she takes her story into her own hands after her encounter with Kylo Ren...yeah she was incapable of failing at what she was doing but it's satisfying to see her character take that step which culminates in her battle with Kylo Ren. Her grabbing the lightSabre from the snow is more than revealing the misdirection that Finn was gonna be a jedi, it's the heroine of the new trilogy finally accepting her destiny. Plus the mystery of Rey's heritage and HOW she is so powerful and how she is linked to Luke is a matter of ongoing speculation and it was one of the biggest questions I had leaving the theater.
On a wider level I felt that all three new protagonists were much more grounded in the Star Wars universe while the Rogue One characters...really it could've not been Star Wars and it would've felt the same. When Rey hears Luke Skywalker's name she lights up in wonder and marvels at the idea that the myth might actually be real. Stuff like that. A star wars fan feels the excitement with Rey at the dropping of Luke's name as being a part of this new tale.
Poe Dameron: I would've jumped into Finn but I wanted to expand on the point of these new protagonists being the vehicles for the audience's immersion in this new episode. It's a very subtle moment but when Poe is brought onto the star destroyer he looks at the hangar with a gaping jaw in awe as the audience joins him marveling at the shiny yet familiar set design. And his scenes with Finn are very bromancy and their dynamic together is entertaining and allows viewers to view them both as likable characters. It's not like they have a ton of scenes together but the screen time they got together was, in my opinion, very well utilized because it made the two of them relatable and it was a more effective means of character development than any protagonist received in Rogue One. And though Poe receives the least screen time of the new protagonists, it's well utilized. Poe is depicted as a confident, super competent, and gallant warrior for the Resistance. Combine that with being quick with a fearless joke in the face of danger, Poe is at least (and quite easily in my opinion) the most memorable and likable of all the "star fighter" characters. Personally I hope we get to see more of him in episode 8 in Han Solo's absence.
Finn: I tend to like Finn a lot more than most mostly because of his arc but even I'll admit his goofiness goes a little overboard at times ("Droid please!" "you got a boyfriend? Cute boyfriend?"). However I really do appreciate his character arc. For all the talk that TFA doesn't do anything new, people really do gloss over the fact that before TFA we never really had a character in Star Wars aligned with the bad guys be so morally opposed to what the villains are doing that they defect to the good side. We've had characters like Anakin Skywalker, Count Dooku (off screen through exposition), and the clones become villains, but never the other way around. You can make the argument till you're blue in the face that TFA was a carbon copy of ANH, but no Star Wars film up until Rogue One with Boddie has had a character arc like Finn's. And Finn does something that only Star Wars Clone Wars had done before and that's humanize the men behind the white masks. In A New Hope there's a pair of stormtroopers making office style small talk but that's pretty insignificant. Finn, more than anyone else in the movie is the image of morality. He sees the horrors of the First Order and can't take part in any of it. But within that there is a character flaw in his crisis of conscious in that, rather than fight against the injustice he refused to take part in, his fear drives him to want to run and get as far away from that evil as he can. It's a central theme to his character that he's a man on the run to the point where Maz Kanata puts it in plain language. And his character arc reaches it's turn after Kylo knocks out Rey. He can either run or do the right thing and defend his friend. And that's what he does. In raising a weapon against a clearly superior fighter in Kylo, he officially stops running and becomes a man of action. From an unwilling hero to an active one.
I like Finn but in spite of being a goofball rather than because of it. And even though the humor is overdone at times, it still carries a level of charm that was severely lacking in Rogue One. Rey is charming in her wide eyed wonder at all she's seeing for the first time, Poe is charming with his humor and bromance with Finn, and Finn is charming on perhaps a lesser level with his good guy act and humor.
As for the rest, I appreciate the nuances and imbalances to the character of Kylo Ren. By every standard across every Star Wars medium of entertainment that I've personally consumed he is an atypical villain and it will be interesting to see where the next two episodes take him. Snoke is just brooding and mysterious. I'm intrigued to find out his backstory so in that sense, like Rey, the mystery of who he is makes him even more intriguing than most of the characters we met in Rogue One. Although you could argue that underdeveloping a character to the point of mystery is a cheap way to get people interested in a character. Maz is just a knock off Yoda. I don't dislike her but she could never make another appearance and I wouldn't really care. General Hux is more memorable than most Emperial officers and his big speech was entertaining, but he's not a highlight character. Phasma is just a waste of a great aesthetic concept.
