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.
WordPress does a lot of things backwards. (Maybe they like to be different?) A couple things that come to mind are...
- They refuse to exhibit clean code standards, and in fact reject anyone's attempts to clean up their code. (Proof: https://make.wordpress.org/core/handbook/contribute/code-refactoring/)
- SVN is tragically slow, and so places like https://SVNHub.com even tell you to use GitHub instead. (Strangely GitHub supports SVN... what's that all about?)
It's pretty depressing when WordPress does everything wrong in those critical points above, but yet they still power 41% of the internet at the time of writing. (Source: https://w3techs.com/technologies/details/cm-wordpress)
So, why does WordPress use SVN instead of Git? To be different! It's broken, but they refuse to fix it.