Other than Bob Dole, no living president, vice president, or former nominee for president or vice president attended the GOP convention.
Paul Ryan was there. He was Romney's pick for VP in the last election.
(I imagine he would have rather not been there, but I'm not a Republican and might be reading him incorrectly.)
Oh yes, I forgot about him.
Well, Al Gore just endorsed Hillary Clinton. The current list of people who have been either President or VP or were the party's candidate who have endorsed Clinton includes Obama, Biden, Kerry, Mondale and Gore. Ironically I can find no evidence that her husband has endorsed her just yet (but I expect that will come).
Trump has Cheney, Quayle, Rubio, Walker, Palin, Jindal, Huckabee, Carson, Gilmore, Perry and.... Ryan. With one of the lamest endorsements by Ryan I've seen yet. notably lacking from Trump are the Bushes (at least three - Jeb, "W" and Sr.), McCain, and Romney.
I'm sure there are others I've forgotten, but there are lots on both sides who either were candidates or ran in the primaries who have given the endorsement. I think it's a mistake to say that either one is lacking.