Reigns at Mania

Had WWE held off on Reigns's announcement until Mania, we'd have been spared another heatless, mediocre match on an already stuffed show, and the show starting with Reigns's music as he surprises the crowd by returning and delivering his "I'm in remission, y'all" speech at Wrestlemania would have been a legitimate Wrestlemania Moment.

…so why didn't they do it that way?

A very legitimate question.  Or he could have interrupted Elias and combined two segments.