They have all the ingredients to be a top 10 team in the world at all levels, be it U18, U20 or senior level. Belarus is one of the very few countries that actually cares about hockey, so in this case I think it is worth pointing out that its population is double the size of Finland so there should be plenty of chances to churn out quality players year after year.
There is a combination of factors why Belarus seems to be improving. It all started with arena building boom in 00's, so kids can pick up hockey in places it was not possible before. Just compare U20 roster birthplaces
this year and
20 years ago. Difference is easy to spot and I think it is safe to assume that more arenas mean more potential talent can be discovered.
I somewhat want to give a tiny credit to Dinamo Minsk getting a KHL team in creating hype around hockey itself and as we see in past couple seasons it has created a pathway for the most talented kids into serious pro hockey. Dinamo helps in keeping few extra kids not defecting to Russia every year as they have local KHL team at home. I wrote a tiny credit because it also helped seriously weakening domestic league which before KHL, even though dominated by imports, was a very serious league in my opinion.
Tying together with Dinamo keeping talent at home, very important is that KHL couple of years ago decided that Belarussian players will not count as foreigners on Russian teams. It is huge because every year there were handful of talented kids picking up Russian passports and refusing to play for Belarus in order better their chances of earning living with hockey. This probably will never fully stop because the line between a Belarussian and Russian for plenty of people seem very thin and we will continue seeing see players born in Belarus wanting to play for Russia NT. Now we some former defectors, like Kodola, Zhuk, Shostak returning into Belarus fold and some will not pick up Russian passports in first place anymore helping out Belarus NT quite a bit.
Equally important is the creation of national development teams (U17 and U18) US style that spends all season together and acts like a hockey academy that plays in second tier of Belarus league system. It also helps keeping kids from poorer regional Belarussian cities not to choose richer Russian academy offers when they come knocking with offers to join them. Just check Belarus U18 results in last 5 years, of course playing together for whole year or in most cases two years, is huge deal and that inflates team's success on international stage, but it also would not be possible with adequate talent and development. Beating Finland, Russia and Czech Republic shows potential is there, soon these results should also reflect in senior NT's tournaments.