Because Linux is just a kernel, it is rather trivial to take Linux, bolt on some programs, and call it a "new distribution." The design philosophy of Linux enables and encourages this kind of behavior.
The major BSD systems (FreeBSD*, NetBSD, OpenBSD, and DragonflyBSD) have all been carefully designed from the original 386BSD codebase. Originally, there were just FreeBSD and NetBSD, based off of the 386BSD codebase. Developer conflicts within NetBSD caused a split, spawning OpenBSD. A similar thing happened in FreeBSD, spawning DragonflyBSD. I will not go into details as you can research that for yourself, but this is how we got from 386BSD to where we are now. Keep in mind that there was a rather large uproar in the NetBSD community before OpenBSD was created; it was certainly not done on a whim.
Most *BSD developers believe their time is better spent working on new problems than re-inventing old problems. Why take 10 developers and spread them out, all to work on their own version of X when together they could collectively build X, Y, and Z. This is not always the case (see the NetBSD conflict which spawned OpenBSD), but for the most part this is the reason you do not see hundreds of *BSD variants.
As I said earlier, it is relatively easy to bolt together your own Linux distribution. In fact, something like LFS can have a complete novice building his own "distribution" in a very short time span. Basically, anyone can make their own distro. What about *BSD? Could a complete novice take FreeBSD, fork it, and make his own JoeBSD variant? Yes, but he wouldn't get very far. Unless he knew what he was doing and had a good knowledge of the code, most of the *BSD crowd would simply ignore his efforts. Not because they're mean, coldhearted, etc., but because they aren't going to waste time on JoeBSD when it is a fork of FreeBSD, which still works perfectly fine.
If JoeBSD brought something genuinely new to the table, people would support it. If JoeBSD was just a copy of FreeBSD with a different package manager, people would ignore it.
*I did not include DesktopBSD or PCBSD because they are not separate derivatives, they are FreeBSD with easy-to-use installers and pre-configurations. Think Ubuntu and Xubuntu.