Because people still aren't, which makes the filter function basically not helpful.
I just realized something else that's an issue if the forums stay merged. You can only put one tag on a thread, so if you tag it with an edition, you can't also tag it "Homebrew" or "DDAL" or one of the others. (Though DDAL implies 5E, I guess, since that term wasn't used in earlier editions.)
...Actual sub-forums would be better