Here's why:
* Exclusive implies a state of being the only one or being limited to a select few. There isn't a "more exclusive" or "less exclusive" state.
* More exclusive would imply a higher degree of exclusivity, which is logically impossible since exclusivity is already an absolute concept.
If you want to express a higher degree of exclusivity, you could use phrases like:
* More restricted
* Highly exclusive
* Extremely limited
* A select few
* Invitation only