I get the impression that both of the cases you cite are driven by dissatisfaction with the existing standard tools.
Perl has been very successful, but it's not without its problems. Hence: Python, Ruby, and even Perl 6. Perhaps I see these projects as a reaction to Perl 5 because I am a Perl programmer, and that's my perspective. Nevertheless, all these project sound to me like someone is doing "Perl, as it should have been."
Revision control is the same thing. CVS's problems have pained projects for ages, and Subversion aimed to fix that. The explosion in distributed RCS seems to stem from Linus Torvalds saying he needs that kind of system to work on Linux. Everybody knew that any system that would work for the mighty Linux would get a lot of attention (i.e., a good reputation). People tried to take the crown from BitKeeper, but ultimately Linus made his own solution (git). I suspect that after some more time passes, a number of the numerous RCS solutions out there are going to pass as well.
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