It's really complex and very abstracted out and hard for people to wrap their brains around, plus (for all its failings, and yea verily there are many) the existing system is pretty well battle-tested and we're wary of making huge changes to it (and also we have enough nebulously-defined, open-ended "let's modernize this thing!" projects going on and are really hesitant to add another), plus it's hard to set up a working notification system in a dev environment so the ability to hack on it is limited to people who like banging their head against fiddly little config bits, plus it's one of the most heavily used systems on the site and so it has major performance implications (so often we have to take the non-obvious but better-optimized route instead, which makes for harder-to-read-and-understand code sometimes, and means that changes have to be scrutinized really carefully for performance), plus there's a ton of stuff chucked in there to work around the fact that Email Is A Terrible Protocol, plus all of the above means there's a certain amount of "sod it, I'm just going to go fix something else instead" going on....

Is there something in particular you'd like to see changed? There are a few frequently requested things that we've deliberately chosen not to implement for one reason or another; if it's one of those I can explain why. (Alternately, it's possible that what's bugging you is a possible fix and I can open an issue for it.)