People usually go to the theater with a group of friends/date to see a movie that (hopefully) most of the said party are interested in. Since bad blockbuster movies get more buzz than your genrally Golden Globe material, more people have heard of it and want to see it.
Plus, usually you go to see movies for fun, and good movies are often rather sad.
By the time the DVD comes out, word will have gotten around about the movie, and if it has OSCAR WINNER or OSCAR NOMINEE on the case, people are more likely to buy it. Plus, some people feel the need to buy "smart people movies" in order to look better. (Oh, you have Dr. Zhivago? / Yes, it is my favorite movie! --has never opened the box--)
This is my theory, anyway. Mostly though, all that advertising that goes into summer blockbusters REALLY, REALLY helps.