CVS was already on the way out even prior to git's inception, because of SVN. SVN's unofficial goal is essentially to be a better CVS, and it was a great success on that front. However, IMO its success made its core dev team complacent.
For example, one complaint I hear often of SVN is that it's slow. Go use CVS for a week -- and I mean not just doing a checkout and make a few commits, but merge, diff, switch branch, add / remove / rename sub-trees, etc.; bonus points for involving >3 people trying to do the same simultaneously -- then try and tell me SVN is slow.
Nonetheless, SVN is not fast, but it's fast enough when there was no viable competition. Then git came along, and even the staunchest opponents must conceded that it's lightning quick. And people will tolerate a lot of BS from a tool if it does the job and does it quickly.
SVN's performance has improved significantly since the 1.4.x days, around the time when git first made its appearance. No doubt some improvements were on the roadmap regardless, but surely having competition lit a fire in getting them out the door.