There have been a variety of reasons and theories behind it through the history of the RPF. The most recent were two reasons. First, as an attempt to slow the influx of sock puppets and second, due to an increasingly unstable software package, due to some server issues and external hacking, in an attempt to maintain what system control we could.
The second did work to a certain extent although moving, suddenly, to a new server helped a great deal also. As of right now we are working on getting to new software, there are still some issues, and once that's done the plan is to reopen registration.