One major reason is the rise of the multiplex cinema. My local one was actually the second to be opened in the country. I remember the excitement that it caused at the time. Ten cinemas under one roof (it now has eleven). Previously cinemas were old buildings in varying states of repair and were frequently quite uncomfortable for anyone over five foot eight. If you were unfortunate enough to sit behind a woman with a large, elaborate hair do, you were lucky if you saw the screen at all. Suddenly you didn't even have to stand up to let someone past, you didn't queue in the rain, there was somewhere to put your drink, you didn't sit knee-deep in ice cream tubs and the projection and sound systems were state of the art. You could even see!
Nowadays, with multiplexes everywhere, the vast majority of the nation is within easy reach of at least one (I have five within half an hour's drive and another one a further half hour). Granted they all show the same films and they are all much of a muchness inside, but the fact that they can offer such a variety of material means that there is almost always something worth seeing on. The film makers, I think, have responded to this increased interest by producing more films of quality than they managed previously. Of course there is still a great deal of dross produced but they are also making stuff that could not even have been envisaged twenty years ago: eg. LOTR., Master and Commander or the Matrix