This issue has been brought up so many times in the past...
The main problem is that the current forum runs on an old version of SMF. Theymos has extensively patched and hacked the code, so upgrading to a new version would be pretty hard for him.
Next to this, a new forum software is being developed for bitcointalk... This decreases the odds of migrating to a newer version of SMF that might be mobile friendly. The new forum script developed for bitcointalk is called epochtalk. The main problem is that epochtalk doesn't seem to move out of beta phase (it's been in beta for yeeeeaaaarrrrssss)... Not much is being developed, and it's just not moving along (at least, so it seems to me)