Why did Commander Riker take so long to leave the USS Enterprise, both in and after Star Trek: The Next Generation? According to Jean-Luc Picard, Riker was one of the most capable officers he had ever met. And yet, for all Picard's confidence in him, Riker seems to have rejected many command positions in order to remain Picard's "Number One."
Picard doesn't seem to have been the only one who believed Will Riker deserved a captaincy of his own. Starfleet approached him several times, offering him the captain's chair on the USS Drake, the USS Ares, and the USS Melbourne. A throwaway comment from Q in Star Trek: Voyager seems to suggest he was even offered Voyager as well. But, no matter what Riker was offered, he always turned it down and decided to remain on the Enterprise.
A major clue is presented in the season 2 episode "The Icarus Factor," which reveals Riker's strained and frankly combative relationship with his father. This seems to have been at the root of Riker's reluctance to leave the Enterprise; he had sought a father-figure, someone to fill that void in his life, and had found it in Jean-Luc Picard. As such, so long as Picard remained on the Enterprise's bridge, Riker would want to be there too.
It's important to remember that a commission to the USS Enterprise was a very prestigious one indeed. This was the Federation's flagship vessel, the ship assigned to the most difficult crises, and it was frequently on the front lines of galactic conflict. What's more, Jean-Luc Picard trusted his crew implicitly, and allowed them to make their own decisions where other captains would have felt it necessary to intervene. While this allowed Will Riker to thrive, it also gave him something of a convenient safety net. The ultimate responsibility would rest upon Picard's shoulders, not Riker's; if the commander made a critical error, then it would be the captain who faced the consequences. In a sense, then, the Enterprise in Star Trek: The Next Generation allowed Riker to develop his creative instincts in a surprisingly low-risk environment.
But there seems to have been one other reason Riker stayed on the Enterprise so long; because he disliked change. Notice that the Enterprise crew only really begins to break up after Worf departs for Deep Space Nine, finally instigating change on the Federation's flagship. That seems to have acted as a trigger for Riker, and by the time of Star Trek: Nemesis he's both married Counselor Troi (Worf's ex-girlfriend) and has accepted captaincy of the USS Titan. At last he's stepped out of Picard's shadow, and hopefully begun to create some legends of his own.