Brooklyn, New York is a vibrant, vital borough of 2.5 million people,
which seems to have few movie theatres, with existing ones closing at
a rapid clip (the Fortway ceased to exist last summer).  What explains
this phenomenon?  Please provide sources, which need not be specific
to Brooklyn.

This is just wild-arsed speculation, since I haven't visited Brooklyn
in many decades. How is the parking situation there? If it is
difficult or expensive to park a car in the areas where the theaters
are located, that might discourage people from going to the movies.

It's possible.  Brooklynites do enjoy driving, but we also have an
excellent public transportation system and the population is pretty
dense (not as much as Manhattan).  The theatre in Sheepshead Bay,
which seems very successful (esp. given the low competition) does have
its own parking lot.  The now-closed Fortway was on two bus lines and
just a few blocks from the subway.  Ditto the Marboro, which closed a
few years ago.

Nelson,

If you're looking for a good theater, you might find some of the
material I gathered for this answer to be of interest:

http://www.answers.google.com/answers/threadview?id=605767