Until very recently, Germany was a loose affiliation of independent states rather than a nation-state in our modern sense of the term. Even Cologne and Duesseldorf, very close together by today's standards, were for much of their histories governed separately (iirc they were only brought under the same political entity when Prussia gained control of the Rheinland and Westphalia after the defeat of Napoleon). This allowed various cultural institutions to develop relatively independently of one another well into the nineteenth century. Combine that with the relative perishability of beer before modern filtration and refrigeration, and beer would not have traveled far from the site of its production, leaving Kölsch, Altbier, Berlinerweiss, etc. to remain locally dominant.

The Reinheitsgebot originated in Bavaria, and its implementation throughout Germany was a condition of Bavaria joining the unified Germany in 1873. I believe that many styles that predate the Bavarian version of the law have been grandfathered in, and the current law is significantly more lax than the nineteenth century version. Additionally, many in Germany today see the Reinheitsgebot as a hindrance to the development of a modern and eclectic beer industry like you see in Belgium or the U.S., for example.