More then likely it is personal preference. Some people like the workflow introduced by SVN, and some just don't like the distributed model of Git. Some like the way commits, branches, tags etc are handled in one revision control over another.
It could also be that they where using SVN before Git was stable enough and they don't want to go through and switch.
Why fix something that isn't broken?
I really don't think there is a good answer you are going to get on this board, unless someone from Automattic is here. There isn't a right or wrong answer in picking version control systems.