As @suchmoon pointed out, the forum’s software has a lot of customizations, not only by modifications made over recent years, but even
as far back in 2014 the same consideration and assertion was made.
What might have happened, and this is mere speculation, is that besides there being a bunch of customizations and a logically heavy focus on security (see concerns on SMF 2.x
here), the more the idea grew to
move on to a new forum software, the less the said compendium of factors pushed for a forum software version upgrade, keeping things stable and under control in the (now long) interim.
Of course, as time goes by, it becomes more difficult to carry out and upgrade, even if one intends to. There may easily be incompatibility issues, usage of mods that are now defunct, depreciated code sentences, and so forth. Not saying it’s not doable, but it’s likely quite a hell of a project itself.