Most of them are kind of redundant frankly or, I would imagine, result in balance issues with the changes to the other races (still haven't played multiplayer yet). Firebats are mostly replaced by Hellions, Medics by the Medivac dropship, etc.
I'll also say that as much as some of the older units can be fun and useful, including them as well as the new units in multiplayer would have been way too many options if you ask me, particularly when some of them are redundant, and others are blatantly more versatile than the new units.