I recently booked flights to Cancun from the West coast of Canada. The flight is about 7.5 hours and gets me into Cancun at about 5:30. With the road transfer to Playa del Carmen and the ferry trip, I can get to Cozumel the same evening. I looked into flying into the Cozumel airport, but it would be about $600 extra each and the flights take almost 2 days, including an overnight 6-hour layover (which when you factor in airport security times, isn't really enough time to get a hotel and have more than a quick nap). Basically, depending on your location, flying into Cancun can be much quicker and more convenient.