well, in part because the US didn't "win" those wars, but also i think because of the effect vietnam could have. There are still fathers who served and still have PTSD (shell shock) and you never know sort of trouble even a video game could cause. I don't know if that would be an actual reason, but for now i'd say its at least a factor in avoiding them.
With that said, there was 1 game i know of about vietnam, "Conflict: Vietnam" The company that made it wasn't the best, and the game was terrible. Mostly because vietnam was a closed, guerilla war. There weren't all that many huge battles like there were in WWII. It wasn't fun getting ambushed on tiny trails, and having to disarm trip lines and stuff like that
And i'd say most people just find WWII more interesting anyways