Main reasons the P-39 isnt "popular".
#1 The Russians were the largest user.
#2 The USA wasnt the largest user.
#3 The P-51, P-47, and P-38 were the main fighters the USAAF flew in the war.
#4 The above a/c replaced the P-39, so the P-39 is considered "old".
#5 The AVG didnt fly the P-39.
#6 The most famous USAAF aces didnt score a lot of kills on it.
...and because of that it gets largely forgotten, which in may peoples minds is the same as it being "no good", ect.
No, its not some uber-down-low-super fighter...and it never was, but it was decent enough vs its opposition, especially the later models P-39N/Q. Just because it wont replace the Spit XVI as the MAs most popular dogfighter, doesn't make it a POS.