*I* think more threaded options aren't available because phpBB and its ilk were "good enough" in that they filled a need for forum software well enough that the owner/operator of the forum didn't have to spend further time on it.
there is NO WAY that phpBB is perfect for everyone that uses it. it's functional enough that the sysadmin can move on to the next thing on the list. that's how crap software lives on; no one has the energy to commit to making something better. discourse really looks good to me, but, again, written in Ruby, so adoption is going to be slim to minimal. also, why would an existing forum operator change when they have a working system?
we all just got far too used to this forum style, and now we're stuck with it.