The Old Mixer wrote:
Riker didn't want to be stuck with a cheesy, thrown-together guest-ship-of-the-week bridge.
Good one! In all seriousness, I think Riker stayed on the Enterprise
because the writers and producers weren't comfortable taking the risk of writing him off the ship for any length of time. The Next Generation
was the first spin-off of TOS, and the TOS films went out of their way to keep the Enterprise
crew together as much as possible (before finally giving Sulu his own ship). The idea that a significant character could be written out of the series or have his role drastically altered wasn't something that had been explored to that point.
DS9 was the first Trek
series to push the boundaries of fans' comfort zones with characters. The crew lost their station at the end of the fifth season and didn't regain it until several episodes into the following year. A regular character and several recurring characters were killed during the show's run. A main cast member from another series was added to the show before the fourth season. Had TNG happened after DS9, it's possible we'd have seen Riker booted from Enterprise
with Thomas eventually replacing him. But that wasn't going to happen with TNG being only the second Trek