Why move it to May? You're mucking up the whole off season calender by doing that.
First off... the rookie OTA's we've already had, those would have to be pushed into a compressed schedule with the already in place June OTA's.
And you'd have the draft going head to head with the NBA Playoffs. The NFL avoids playing games during the World Series. Are they going to do the draft during the NBA Finals?
If anything, the draft should move closer to the end of the season. Hell... make it the week after hte combine wraps up.