I don't know why, sorry.
But I do know that because of the current illiterati making the word forums common language it is now correct to use it in place of fora. Many other words have gone the same way.
Recently I complained to the BBC about a newscaster using 'stadiums' (
) and they pointed me in the new, correct way as featured in the latest edition of the OED.
It's only going to get worse.
Still, you can't help but
lol
Sorry.