Actor Wil Wheaton wished to pursue an acting career.
Also, lots of people didn't like Wesley.
Kitface 11:06, June 14, 2011 (UTC)
Season 7 provided closure for the stories of some recurring Starfleet crewmembers. In Wesley's case, he suddenly entered a teenage rebellion phase and objected to a forced relocation project found necessary by Starfleet in the episode Journey's End .
That was when The Traveler unveiled himself and resumed his part in Wesley's guidance. He helped Wesley to decide against continuing his Starfleet career in favor of transcendental pursuits which were arguably more worthy of his talents. After seeing the forced relocation project cancelled, he says his goodbyes to the crew and departs with The Traveler bound for "other planes of existence."
Note that Wesley can be seen attending the Troi/Riker wedding in Star Trek: Nemesis. Apparently, his extraplanar travel powers include the ability to return for important social functions.