Tampa Bay was willing to give Nash nearly $100k more in a guaranteed money. Baltimore wasn't going to pay that much, but did give him a better chance to earn a spot on the roster. Their depth chart is a lot less crowded in the middle. He's basically taking a bigger pay day earlier in case he can't hang on in the league.
As for Nash the player, his FA slotting is really a reflection of the modern NFL. Nash is a terrific run stopper. But the NFL needs their run stoppers to offer some sort of interior pass rushing threat. Nash needs to prove it more vs. NFL caliber linemen.