Or at least one possible reason.  I'm sorry I can't find the link to support the following argument, but it's late and I don't feel like looking too hard.

That said, I definitely read somewhere over the past month that Coach Schwartz (and most other coaches) were looking for three down players.  That is, defensive players who could stay on the field in running and passing downs.  The article made some sort of breakdown of the number of snaps a 2 down player vs. 3 down players actually plays over the course of the season, and it reveals that (surprise!), three down players, while they're paid about the same, play about 50% more snaps, thus making them a much better value.  Why do I mention this?  Because Rey Ma was the ultimate two down player.  GREAT against the run, suspect in coverage.  This from his Pro Football Weekly scouting report:

"Lacks lateral agility and is not fluid dropping in coverage.  Can be exposed in space...takes some questionable angles...did not play much man coverage and can be outquicked by fast backs."  Plus, regarding his maturity, it says that he "still requires extra monitoring and has shown questionable judgment.  Would be best in a strong locker room where teammates hold him accountable."

So that's two big strikes against him.  I guess the Lions figure that Delmas will help the run game and pass game, but ReyMa would only help the run game.  I'm not sure I 100% agree, but at least I understand.  Cunningham will never have to take Delmas off the field, but ReyMa would be sitting out any nickel or dime formation.  Ultimately, they couldn't rationalize a pick this high on a part time player.  Plus, I don't know for sure, but I would guess that the Lions do not have a particularly strong locker room to keep him in check.  Who would provide the veteran leadership?  Sims?  Hanson?  Seems unlikely.  I'm sad that we missed out on a potential defensive stud but I think it'll be OK in the end.