Why did the Berlin zones join together?

First of all, please note that all four Berlin zones did
not join together, at least not until Germany reunified in 1990.  The zones administered
by the three Western allies (the US, Britain, and France) did unify in 1949.  They did
so because the Western Allies wanted a stronger West Germany to help protect against
communism.

The Western allies were very concerned about the
possibility that communism would spread across Europe.  They believed that "their" parts
of Germany would be much more likely to sympathize with communism if they continued to
keep those zones separate.  They felt that putting the zones back together would make
Germany stronger economically and make Germans happier to ally themselves with their
former enemies.  The US, in particular, pushed for this to be
done.

The USSR did not want this to happen and blockaded
Berlin to try to prevent it.  The Allies then mounted the Berlin Airlift.  Soon after
the Airlift caused the end of the blockade, the Western allies joined their zones in
Berlin and in West Germany.