Why was what they shown not enough?
Kylo Ren was demonstrated to be a mentally compromised character, trying desperately to show the same traits Darth Vader showed, but failing when facing adversity. He was able to get a handle on Rey and take her to the brink initially (all the while being injured from a Bowcaster), but failed to finish the job because he thought he was strong enough to be able to turn her, letting his guard down and putting himself at a severe disadvantage the entire fight because it gave Rey the chance to recoup and really believe in herself and the force.
I don't see how you can see what unfolded and think it wasn't made clear enough. I really don't.
Because how you present information is what determines how that information sticks to an audience.
It doesn't matter that the cave troll says (TWICE!) that Kylo Ren's training is incomplete, they didn't explain it, and it's a throwaway line in two different dialogue scenes, it's not going to connect with the audience.
It doesn't matter that you can infer that the crossbow wrecked shit earlier, therefore a glancing direct hit will slow Kylo Ren down if you don't actually show that having an effect, or have the crossbow wreck shit in what comes off in a throwaway gag (which is why a bunch of people failed to make that inference).
EDIT
(Full Disclosure: Again, I didn't have a problem with how the fight went down, I only observed that there was ferocious debate about it on here and on other webzones: but with that said I 100% failed to make the connection between the glancing hit from the anti-tank weapon and it slowing Ren down until I read an article about it later)
It doesn't matter if you make him mentally anguished if you put him in a situation where that mental anguish isn't exactly applicable (OR SO WE'RE TOLD!). This is the difference between this fight and the fight in RotS, it's established that Vader doesn't want to fight Luke, he really wants his son to join them, he is conflicted about it which is why Luke defeats Vader this time. Kylo Ren is broken up about his dad's death (even though he killed him?), but why would that immediately extend to his fight with Rey?
Basically, we have a bunch of people who missed the point of the ending. Part of the problem is because we, the audience, are given 4 reasons of varying definition and clarity rather than one specific reason.
For example, if it's explained that Kylo Ren was never properly trained in laser sword combat- so therefore he's not an unbeatable wrecking machine at laser sword combat when compared to someone who has never picked one up before last Thursday.
And I really don't see the inherent problem you have if the movie had stopped and tried to be clear about one of it's plot points.