[JeremyReimer]

phpBB added a mobile theme that works pretty well, but many sites didn't bother to upgrade.

Mostly this was because the upgrade was difficult and required multiple steps (first updating to the latest sub-version, then to the major version, then to the latest sub-version of the major version) and if anything went wrong in any of the steps you could take down your board.

And if you had any custom code, as the forum operators at Ars Technica had, it was just too difficult and too much of a time investment to even attempt.

So phpBB, which was one of the most dominant forum software applications when forums were at their peak, became known as "old-school" and "non mobile-friendly" when that was technically not true but it was de-facto true.

The Ars Technica forums today still exist but they are difficult to view on mobile, and as a consequence their traffic has declined significantly in the last ten years, despite traffic to the main site increasing.