DR flights aren't as concentrated as in PR. Over there LMM probably gets over 90% of all flights to PR. The DR has the busiest airspace in the Caribbean, but in most Caribbean islands almost all the flights (in many cases all the flights) is handled by one airport. If PR received more flights than the DR, then its airspace would be busier than the DR's.
Also, most flights to PR are from the US mainland while the DR gets a lot from Canada, Latin America and Europe in addition to the ones from the USA.
It also appears that in most flights to PR, most of the people are Puerto Ricans and Puerto Rican-Americans (to use a term to refer to those that were born and grew up in the US mainland.)