The God Of Procrastination wrote:
which really makes me wonder why the forums are going
Because MediaWiki wasn't meant to create something like forums, it had a lot of issues under the hood and it had limited portability. Fixing it would cost more resources than it was worth, so it was no longer viable. Yeah, it's annoying that wikitext is not supported, but it has tags and limited WordPress-style markup, which can get a lot done. The main hiccup is wiki technical questions; if we're ever getting some sort of wikitext compatability, I would expect it to be probably in a box or something, like <pre> or <code> tags.
Probably 9/10 discussions can be perfectly fine without any wikitext.
"Under the hood" could refer to any number of things from the specifics of how the code was written to deliberate decisions that make it incompatible with newer versions. Sometimes, the phrase can be used to mean not necessarily a technical limitation but an inherent inefficiency. That certainly seems to be the case, at least in part, with Forum. Forum, along with all other threaded discussion methods (article comments, blog comments, and message walls) store content in a manner which is viewed as inefficient and cumbersome to work with. On top of that, the features are not compatible with mobile devices.
Discussions vs. Forum has been discussed for quite some time; ever since Discussions was released. If you want to know more, the latest thread can be found here.