Hmm, really?

It's hard to measure objectively... but my perception was always that there was an explosion of SCMs around the early-mid 2000s, after everybody got sick of CVS, but eventually Git won out and a lot of the alternatives / diversity in the space went away.

Within short succession, there was Subversion, Arch, Darcs, Quilt, Bazaar, Mercurial. On the commercial side, perforce had a certain amount of traction, and the Linux kernel was using BitKeeper... which no doubt encouraged a lot of the innovation in the open-source space.

Then, once Git was developed, got huge exposure and traction through the Kernel, and GitHub came onto the scene, it quickly became dominant - despite the alternatives, and despite the goofy interface.