Geordi was apparently the only one who knew how to work the promotion committees.
He kinda failed upto Nemesis after that though!
(Would have been funny if he'd kept getting promotions every season/film until Nemesis we have: Fleet Admiral Geordi La Forge - Chief Engineer!)
I think starfleet probably just caught on after the second promotion. They fired his inside man/woman on the promotion committee and figured it would be better just to keep the whole thing quiet as long as Geordi was doing well in the chief of engineering position.
In Voyager's "Timeless" we see LaForge as captain of the USS Challenger. The only reason he's not captain now is because Harry and Chakotay frakked history!