Mainly because perforce is ultra reliable and has a myriad of different tools that already work with it. I've used things like mercurial, CVS and subversion (in fact we used subversion before we bought perforce) but they just don't cut the mustard on reliability and performance. I personally haven't tried Git but the overall cost of perforce is low compared to the peace of mind you get. We aren't stuck on anything and it's possible this could change in the future.
My last three jobs I used Perforce. My current job I'm using Subversion. If I had it, I would gladly pay $10k to switch us to Perforce. - Superior tools - Super fast, even when servers are in different countries - Sensible branch and merge support We've tried Git, and it fails in the first two, especially for machines on slow connections.
For our uni project we are using SVN and for the scope of what we are doing it is fine. I have used perforce (not much though) when I was doing some part-time work at a local games studio. One thing I didn't like about perforce was that the very first time I synced from the server it was just hanging on a .bat file (like 2kb) and the reason was because the HDD it was syncing to was write protected. It just hung there as if it was a massive file and never gave any errors or anything. I am not sure what version it was but it may have changed in the latest version.
I've never seen that issue with P4. It's usually very solid. I've shipped games with SVN before, perforce just has an edge.
