After he basically saved the Federation and Earth particularly from Borg assimilation, I think Starfleet Command just decided to let Riker do what he wanted. If Riker wanted as his reward to go on being Picard's yes-man aboard said cruise ship, then that was what Starfleet Command was going to give him.To be fair, Picard ran the 1701-D like a Cruise ship the majority of the time.
(And besisdes Riker turned down a Command of his own to first serve on Picard's cruise ship; and passed up two more offers on screen -- and maybe one offscreen after the TNG episode "Best of Both Worlds" situation.)
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