Due to a couple of reasons:
If someone wants to write a patch for it, I will seriously consider adding it. I believe that safely adding 2FA would be very time-consuming, so I'm not willing to do it myself or direct Slickage to do it.
2FA is apparently fairly difficult to integrate into SMF (the current forum software).
It would be good idea to have some extra layer of security like 2FA. I am only junior member but I want to feel secure that I will not loose my account easy. I can imagine some guys live from their accounts in here.