Why is there such a difference? Well for starters also look at the number of ice rinks in Finland and in Slovakia. Despite the similar population Finland has 268 indoor and 55 outdoor rinks compared to Slovakia's'62 indoor rinks and 17 outdoor rinks. 10-15 years ago the difference was even bigger as Slovakia was closer to 40-45 indoor rinks back then. That automatically means a smaller amount of clubs and hence also players.
But additionally even the fewer clubs that we have are a lot of the time short of the players, especially with the smaller teams they often struggle even to find 15 players to ice for games in leagues in low age catgories. And even the bigger teams don't have enough players to create good competition between them. Some people in Slovak hockey like to complain that it's all because of tablets, PCs etc. Which I think is just an excuse as those things exist in other countries as well. Kids change and you need to adapt to that instead of complaining about how you could make a selection of the best players back in the 70s and how you have to actively recruit to get enough players nowadays,
And that's a major factor at all youth levels that there isn't a very competitive envrionment which onviously isn't the case in Finland.
I was just thinking abut this when I watched the compeition between Petra Vlhova and Mikaela Shiffrin in the last few slaloms. Petra Vlhova is extremely disappointed to finish in 2nd place because she didn't beat Shiffrin which in turn motivates her to try harder in order to beat her next time like she did today. She doesn't want to finish 2nd, she wants to win. On the other hand I kinda feel like Slovak hockey and other team sports tbh don't have that mentality. I mean just look at what the coaches say ahead of WJC and UHWC-18 - the goal is to avoid relegation and make the QF (before when the sytem was that only 6 teams making the QF it was 1) avoid relegation 2) QF is a bonus, they are just lucky that now if they reach one they also automatically reach the other as well). I know you have to be realistic, but that doesn't mean you should have a loser mentality,
That is of course just one of the reasons as there are many others - there is definitely more money in Finnish hockey, their coaches throughout the system probably aren't stuck in the last century, Finland as a country overall is less corrupt than Slovakia etc.
Fun fact: Finn Jukka Tiikkaja is involved with the Slovak federation at the moment as part of a group that is creating a dvelopment plan for Slovak hockey