You could try doing some basic research in a library and/or Google search and check up on any of the various books on English horror movies of the period all of which give various reasons for Hammer's decline.
Denis Meikle, Jonathan Rigby, Wayne Kinsey, Marcus Hearn and David Pirie all discuss the decline in their respective books.
Bottom lines - no American money any more; economic recession causing collapse of what was left of british film industry; The Exorcist; failure to adapt to chnaging market tastes.
Take your pick.
This so-called "revival" is nothing to do with their output from the 1950-1970s. The Hammer company doesn't even own the rights to distribute these movies.