Not to mention another probable reason: Mediawiki's codebase is a mess and should be rebuilt from the ground up, if possible not in PHP. I once had to build an extension for it and it still gives me the creeps.
I'm not a fan of PHP as a language, but given the community has been working with PHP for 16 years, it would be odd to switch suddenly to an entirely new language and expect support and adoption.
The codebase is an absolute nightmare though, and a ground-up rebuild would be great. I wonder though about it having a similar affect to the Wordpress codebase: people who recognise the mess stay away completely, and people who don't contribute, leaving a contributor base who isn't really equipped (or extremely willing) to do a quality, maintainable rewrite.
I suspect any rewrite attempt would be doomed to end up being an unmaintainable behemoth.
A better approach would be to focus on secure integration tools and API entry-points, to make users less entrenched and solely dependent on the MW software.
