Bingo. Tracking down a REAL AK-47 in its original format (not a semi-auto or parts gun converted) is very hard to find. Then throw in the other multiple effects:
- 1968 ban stopped new ones coming in to the US
- they were Soviet or Comm bloc weapons and as such the chances of obtaining one during height of Cold War was VERY small
- majority were from Vietnam vets as bring backs, and to a lot of collectors 'provenance' adds a premium
M-16's on otherhand were far easier to come by (relatively speaking). There are variants like a M231 port firing version for US APC's that is very rare, and some other variants, but overall its just a simple matter of supply and demand. If you were to remove all the references to AK-47's in movies during the past 30 years, I bet their value would drop just from that alone. People see owning one as a 'cool' factor.
Doesnt hurt that ammo is easy to come by too - MP44 might be pricier if ammo were easier to obtain. But thats just speculation.