Because the very tone and idea of the movie would be completely erased.
It's all very well to say all movies after such a POD are butterflied away, however that's boring, it's more interesting to describe how well known movies made soon after are changed instead.
Saying alternate movie ________ goes on to win an oscar doesn't mean much to us unless you provide us with a screen play, while altering key plot points, background and character motivation of well known movies & characters allows us to identify with them and gives us an insight in the alternate society that made it.
For instance with Top Gun, I'd have Maverick winning that title in the opening of the movie. He's cocky, brash and made it through the Navy's Fighter Weapons School without any repercussions for his recklessness. But what does being "Top Gun" in a world that suddenly explodes into WWIII? Nothing, it's a peace time accolade. Goose gets shot down and killed because of Maverick's glory hound behavior and it is here that he has his crisis of confidence. At the end Maverick goes on to redeem himself in the decisive battle against the Black Sea Fleet.
Same characters, same basic story arc, but a completely different movie.
Other movies that could be altered off the top of my head.
Rambo II is going to have a completely different plot, he'd be released to fight in the War rather to rescue POWs in Vietnam.
Predator would have all the moral ambugity stripped from it and the Predator may be packing heavier weapons, what with a full blown US invasion taking place in Central America.
Die Hard would have McClane fighting Spetnaz commandos on the first day of the war.