because its highly optimized with a heavy use of cashing, phpbb2 is a major version behind, means I can update both hhah and shah on the same codebase, and beyond the huge performance gains there's what is commonly known as a 'fuck tonne' of new features.
HHAH has been on phpbb3 for nearly a year, shah wasn't upgraded because there were no scifi theme friendly skins available. The smartdark theme I made myself for hhah and it took weeks to get it tweaked.
Things like subforums, etc. are vanilla phpbb3 features.