Because Ford didn’t have the delegates counts necessary to be presumptive nominee. Reagan was in a floor fight.
I know that, some are saying Reagan didn’t endorse Ford as if it is the same. Not at all the same and Cruz is certainly no Reagan.
If Cruz had not agreed to support the nominee and would have kept his attitude in check he could have pulled this off without endorsing Trump. As it has turned out he would have been better off if he had not given a speech at all.