"Back in April of 2017 the task of the Enterprise making an appearance came to be and work was to start right away. The task started with the guideline that the Enterprise for Discovery had to be 25% different, otherwise production would have most likely been able to use the original design from the 60’s. But that couldn’t happen so we took Jefferies’ original concepts and with great care tried to be as faithful as possible. We had the advantage of a ten-year gap in Trek history to retro the ship a bit with elements that could be removed and replaced somewhere in the time frame of Discovery and the Original series. […]
After Enterprise, properties of Star Trek ownership changed hands and was divided, so what was able to cross TV shows up to that point changed and a lot of the crossover was no longer allowed. That is why when JJ [Abrams]’s movie came along everything had to be different. The alternate universe concept was what really made that movie happen in a way as to not cross the new boundaries and give Trek a new footing to continue."
It's all down to legal reasons, reasons which now rule out any future crossovers.