You answered your own question tbh, money is a big factor.
However if I remember right the Russian League is getting tighter on the inclusion of foreigners in line-ups. I believe you currently need at least 6 Russians in your team at any time, with plans to make that 7 in the coming years.
No laws either so you can have as many as you like and they don't need any caps or owt. Shakhtar have specialist scouts tracking all over Brazil though, got 3x 17 year olds over there or something, was reading a good article on it today. Best to buy in bulk so they keep each other happy.
BTW their kit is fucking lush.