I would say the concentration of Czech/Slovak people around .NET at MS is a result of accidents and networking. I got into the team thanks to a friend of a friend who heard I was interested. But yeah, it's a surprising number given the size of the population of Slovakia and Czech republic.

There's a newly opened .NET development center in Prague so the number is likely going to grow (they do hire from all over the world though): https://karelz.github.io/hiring_prague_net/.