Most of the different forum softwares nowadays are not made for mobile devices. Actually, it's no technical problem to let a website look and behave like a dedicated app for a mobile device. But in order to do that, you'd have to design your software from the beginning with mobile users in mind. Everything else will take quite some work afterwards.
Heck, this forum is designed with 4:3 displays and low resolutions in mind... Version 2 came out in 2008 and that version is not too different from version 1 which is even a few years older.