Another reason we don't see myrms in Myth II, some have theorized, is because when Balor died, his spell of immortality died with him, and the myrms all died as well. (That was prolific use of the word "died".) Personally, I don't subscribe to this theory, 'cause I like to think that cool units like myrms are still alive somewhere, but that doesn't mean it's not worth mentioning.
And one of my own theories as to why they didn't appear in Myth II: if you look back at the maps for TFL, bungie used generous helpings of myrms in those maps...so it's possible they decided the audience had seen enough of myrms after TFL, and simply phased them out while replacing them with units like krids.
-Joy
