Why Dagestan produces so many professional fighters?

I've seen people asking this or talking about this in numerous threads. Often many incorrect assumptions are made so I'd like to give my own take on it. Which might be more credible to some since I'm from the same region.

I've used Dagestan as main example because it is the largest republic from the Northern Caucasus. But this applies also to Chechnya, Ingushetia, Ossetia and so forth. The Southern Caucasus countries also produce lots of great fighters probably for similar reasons, but I...

Why Dagestan produces so many professional fighters?