How is this the 50th Anniversary Collection if it doesn't include the iconic Symphony of the Night? That belongs in the collection celebrating the anniversary, not some future collection.
So, I think there is a pretty dramatic shift from the classic style of Castlevania to the metroidvania style that emerged with SOTN. It sorta makes sense to put the metroidvania games in their own collection.
That being said, Rondo is a huge omission, though I imagine that is because they just released it.