Firstly, because contrary to popular belief, the film going on screen for cinemas is on tape and not in dvd format (far worse quality). It takes time to convert the tape into dvd format, the film producer has to get the rights, get them passed on to retailers, market them, etc, etc. Also, pirating is way to widespread these days so as soon it comes on the market, it gets ripped and people will stop watching it on the big screens. Today's film revenues are still made by cinema screenings and not dvd sales.
They have to see how popular the film is before they decide how many copies of the DVD to make. They're also not going to know from the start how long it will be on the cinema for.. so it takes that much time for the shops to put their orders in, then for the promotion staff to sort out posters etc, and for the things to be made and shipped. 3 months isn't that long really.
The distribution companies have a set time frame for transfer from cinemas to DVDs. That's why a few months ago Odeon announced it would boycott Disney films because they wanted to reduce the time it takes to release DVDs (but they sorted everything out and then everyone was happy )
(Original post by Rubgish)
If they released the DVD too soon people would be less likely to go to the cinema. If I knew i'd only have to wait a few weeks for a movie, i'd save my £5 and just wait for the DVD.
Me too, except for the films that only work with incredible surround sound. :P