Lesnar v Cena happened at Extreme Rules of all things. I don't think WWE really care at the moment.
Yeah but that kind of made sense (or would have, if the result had been different); Lesnar debuted the day after Wrestlemania in 2002, so they wanted that symmetry, I think - having him return 10 years after his debut.
Plus, the PPV after Wrestlemania has always been one where they give us something quite big and monumental; as far as returns to the ring go, they had Goldberg's first WWE match in 2003 and Hulk Hogan's return to WWF (teaming with Shawn Michaels) in 2005 at Backlash.
As far as historical matches, the PPV after Wrestlemania has a pretty long history as well; for example, Hulk Hogan historically regained the WWE Championship in 2002 after a long absence from the company; The Rock finally took the Championship from Triple H in 2000 etc.
As far as Lesnar returning for this PPV, and facing Cena, I don't have a problem with. The only problem I have is that they've already had Lesnar lose (in his first match back). Seriously, who loses their comeback match? It just seems stupid. I'm guessing that they've got something big planned but, for the life of me, I can't understand that decision.