1. Once A&C MEET F (and every other monster) happened, Universal seemed to see the monsters as silly things that were not to be taken seriously.

2, Studios tend to not appreciate that people will see a good movie. Make a couple bad horror films that lose money and they see it as the fault of the genre and not themselves, so their egos pronounce the genre dead and move on to other genres.

3. TV started to happen and theater audiences took a nose dive. It took a little while for studios to come up with a new game plan. To combat this Universal moved on to the 3-D craze and The Creature series.

It is interesting to note though, Uni didn't completely forget its monsters, they just moved on to mostly new ones maybe thinking the old ones were stale.