Obviously we have all seen Ratimov around, but I have no idea who D-oN_ is (2nd on the list in terms of activity). Looking at the first 5 pages from his post history, all his posts were made in the Russian boards.
It's the same thing with the 3rd most active Russian - The0ldl_lser. Not a single post in English in the first 5 pages of his post history.
Azrieli has a few posts in English, but the majority of his writing also takes place in the local sub.
I bet there are many Russians who don't even speak English or write in the English boards, but they are very active on Bitcointalk and get plenty of merits in the local board alone.
You are absolutely right, there are many Russian-speaking users who rarely leave the local section, but I think this is the paradox of all local sections. But the most interesting thing is that the Russian section would be much more active if there was not such a mess as it is now. Against this background, many people lose all desire to visit the Russian section, so many Russian-speaking users live outside their language section. The situation is the same with merit, I will not go into details, but it is much easier for Russian-speaking beginners to advance in rank than beginners who speak English or any other language.