Amazingly, they were able to sell their own highly specialized brand of horror for a good fifteen years, till the well ran dry, or more properly, they stopped drawing from it.
Hmmm... Univeral Horror: 1931 through 1945 or 1946 when it peters out (prior to A&C) - about 15 or 16 years. Hammer Horror: 1957 through 1974 or thereabouts when it too sputters to a close - about 17 or 18 years.

Maybe that's the basic answer to "what happened?" Things do have a shelf life.