Why? Those AK's are hillarious.
I was talking with a military personnel friend who is thinking about joining that unit and we also laughed a lot. BUT at least they tried
The reason that is probably behind the decision for choosing that particular handguard (Ukrainian made Crook/Крук) is the weight. It haves that pattern that is full of holes and still adds some functionality but you better don't hit to much that thing on hard objects and also when you shoot you have to do it with gloves otherwise your hands will cry from the temperature. We can also see the non standard muzzle compensator use, the lack of using electronic sights, use of bakelite magazines etc. They buy a lot of the gear from their pockets.
Also the price is a big factor here as the budget of the Bulgarian Joint Special Operations Command is pretty low with slight increase in the recent two years. The government is pressuring them to use locally made Arsenal AKs but maybe half of their inventory is outdated (Sovyet made) and only slight amount of AKs received upgrades. Now at least they have a good number of MP5s which they use extensively in training because the ammo is cheaper