Even with centrifugal "gravity" the toilets need to be designed for the worst case scenario (no "gravity"). Even if you could use a "regular" toilet the system needs to sequester and process the septic waste. That precludes even using the likes of an airplane toilet.

It's a significant amount of engineering effort, testing, feedback, and iteration to build effective life support systems for manned spaceflight. Long duration spaceflight is orders of magnitude more difficult.

Toilets are systems that can incapacitate or even kill the crew if they malfunction. In a low or microgravity environment aerosolized septic material can get in astronauts' eyes or lungs. It can also seep into electronics or other ship systems causing malfunctions. Even just clean water spraying into the cabin could be dangerous in microgravity.