So - Riker finally accepted a posting on Titan. Why did he give up the dream of commanding an Enterprise? Official decree from above - "take a ship or you're out!" - or a decision to move on combined with a ship that he fancied as equivalent to an Enterprise?
Who says he gave it up? Not everyone lands his or her dream job at the first attempt. Riker probably thought that he would stand a better chance of getting the Enterprise
by finally distinguishing himself as captain of another vessel. The idea is that when Picard finally stepped away from the ship that Riker could use his experience as Titan's
skipper to bolster his claim on the vessel. That he would already have ties to any remaining Enterprise
crew wouldn't hurt.