Speaking as someone who works for one of those companies, I can give you a background into the reasons why we chose and continue to use Perforce.
First, some background. The company had grown over the course of it's first three years to a two-site, 100 person company. Up to that point, we had used CVS with a bunch of bespoke scripts to handle branches and merges. We used GNATs (I _really_ wouldn't recommend this) as our bug tracking system. These decisions had been taken somewhat by default, but by 2005 it was clear that we needed something new that would scale to our needs in the future. Our requirements were
a) Integration of bug tracking and source control management. For us, we felt that it was vital to understand that SCM is only half of the problem. I think that this tends to be overlooked in non-commercial environments.
b) Scalable to multi-site and 100s of users.
c) Ease of use.
g) Windows/Linux support. We're pridominantly a Linux shop, but we have teams who write Windows drivers.
We looked at the following systems (in no particular order):
a) git. Git had been active for about 6 months when we started looking at it. We liked the design principles, but at that time there was no obvious way to integrate it into an existing bug tracking system. It also had no GUI then (although I'm a confirmed command line jockey, a GUI for these things definitely improves productivity) and there was no Windows version of git. Finally, the underlying storage was still somewhat in flux, and all in all, it seemed just too young to risk the future of the company on it.
b) Mercurial. Many of the problems we had with git also applied to Mercurial. However, even then it did integrate with Trac, so we could have gone down that route. In the end, like git, it was just too new to risk.
c) Clearcase/Clearquest. Too slow, too expensive, and rubbish multi-site support.
d) Bitkeeper. Nice solution, but we were scared of the "Don't piss Larry off" license.
e) Perforce/Bugzilla. Provided "out of the box" integration with Bugzilla, worked pretty well with multi-site using proxies, had a nice GUI, scaled well, was stable (our major supplier had used it for a few years), had client versions for Windows and Linux, and was pretty quick, too.
f) MKS. No better than CVS.
g) SVN. In many ways, similar to Perforce in terms of how it is used. In fact, one part of the company decided to use SVN instead of Perforce. However, this lasted for about 6 months. I don't know the details but due to some technical difficulties, they gave up and moved over to Perforce.
All in all, Perforce integrated with a customized version of Bugzilla, while not perfect (git/mercurial/bk's model of how branches work is more sensible I think), gave us the best fit to our needs. We now have ~200 users spread all over the world, with no real performance problems. The bug tracking integration works well. Perforce's commercial support is responsive and good, we've never lost any data and we can tune the whole system to our needs.
If we had to revisit the decision, it's possible that Mercurial/Trac would have fared better, but to be honest the system we chose has stood the test of time and so there is no reason to change.