The Mirelurk King wasn't genetically engineered, while the Cazadore was. Swamplurks are seen in Point Lookout and are said to be from a different creature altogether.
I would agree that they shouldn't have been in New Vegas either, but from Obsidian's short dev cycle they used Bethesda's resources in the engine. Besides, if there are several types of "Lurks" on the east coast, it's possible for there to be others around the U.S.
I don't think Cazadores should be used in Fallout 4, but I wouldn't be against similar creatures. I think that's why they incorporated Blood Bugs though.