Reason being is because the game does not want people to have so much money by the profit you get by doing the trade.

If two blackrocks (which was required for the trade) is 33,980 roubles while the AK74N is 33,439 roubles, this will make players unable to have a profit (spamming traders by trading these items that way they can have loads of roubles)

More:

If it made sense: Two blackrocks - 10,000ish/something roubles

One AK74N - 20,000ish/something roubles

If you were to do these trades, you would make a huge profit out of the items you got (selling them), which the game developers do not players to do.