Tried to reply to welcome thread but got permission denied.

Anyway, what I was trying to ask is: why not an existing software like phpBB or SMF?

Many reasons. (Although that decision did not come lightly)

We need very peculiar and specific integrations with our account system. Same for squad boards.

Competent forum software is also very heavy-weight, i.e. all code for this game is a drop in the ocean compared to phpBB; we're running a tight ship here.

Keeping up with updates (and popular exploits) WRT re-patching our integrations back during every release is also very unpleasant.

Plus, this small little forum is sweet :)