Because it was written that way in the storyline. In the last STNG movie, it was mentioned by Picard that Riker was going to his own ship. The writers for the Titan Series of novels took it and ran with it. Picard was inferred to retire from command of Enterprise in a couple of years. Riker was going to help break in a multi racial crew to a starship- something bigger and more versitile while introducing other races from Starfleet to see how they can use their talents and skills.