Because heavies make more money, so promotors are all scouring Russia and Ukraine looking for the next Klitscko instead of the next Tsyu.
You dont know what your talking about, check amateur rankings, russians have great boxers in all wieght categories. The kid in my avatar is actualy number 1 p4p in amateurs, just becouse their smaller guys dont turn pro doesnt mean they dont have talented boxers in lighter wieght classes. Different question should be raised, why those talented guys dont turn pro ? you can see them taking medals in most of worlds major tournaments (amateurs) but theres only few who turn pro. I persnaly think its similar situation like in Cuba, goverement has more controll over their fighters compared to other countreys, Karmazin is good pro boxer but then again he lives in US.
You raise a good point, I'd say that former-Soviet nations are 2nd to Cuba when it comes to Amatuer boxing. Like Oleg Saitov... great Am, but never went pro.