Personally, Frankenstein as a film was a massive leap for Universal that helped push the Monsters to become a massive brand for Universal Pictures. Seeing it get the main face as opposed to Dracula, is understandable in a few ways (to not confuse people with Hotel Transylvania, especially if most of the Universal Monsters will be represented); and it fits Murdy's love for the creature and said film.
Not to mention, it can be easily represented as a main attraction; with the vehicles being one of Dr. Frankenstein's newest creations (especially as it's seemingly being the 2nd gen of the Kuka Robocoaster system).