That's why this thread exists.
I'm not too sure if there was any canon datafax about this, but from what we've gathered so far, it is because the Modern Belka system is a Midchilda emulation of an old, dying but niche-use school of magic, which in this case, is the Ancient Belka system.
As a side note, with the recent Jail incident, I believe tactical analysts will think more favorably of Belka auxiliaries, since the Belka emphasis on personal strength made them more resistant to the "cheap" AMF.