The older version of the forum software BEFORE quotes got broken, was very unstable/slow and crashed the server VERY often. The upgrade to this version made it much faster and stable, but broke the automatic quoting system, and a few other things. This broken forum is a HUGE improvement to how it was back then crashing all the time.
There was a push by the Institute for new forums in early-mid 2011. It failed miserably.
There was all sorts of discussion/input/complaints throughout last year. Here is a topic that I quickly found. Israel Curtis mentioned some of the reasons for not moving to the more popular forum softwares:
https://forum.freecapitalists.org/t/my-thoughts-on-the-forum-software/18668
The Mises Institute most likely puts the forums at the very low of the totem pole due to monetary constraints.