People are relatively confident in Miranda because last year's breakout came with increased contact skill, bat speed, and ability to drive the ball. Those things don't really "go away"; the hope behind Miranda is largely scouting-based, not necessarily stat-based.
Obviously, he could still fall flat on his face against major league pitching but that applies to any prospect.
With that said, some are all too willing to directly translate MiLB success to the expectation of immediate MLB success. It usually doesn't work that way, which is why smart front offices don't routinely bank on unproven prospects, especially at multiple positions at once.