Here's the problem a coach runs into. Ty was showing those blindspots in scrimmages as well. Yes, there were scrimmages he did very well and even outperformed Milroe. But he also showed enough of his blindspots that didn't give the staff enough confidence to put him above Milroe. Again, what tilted the scales for Milroe was his ability to throw the deep ball and his speed. If you take just one of those things away and Simpson probably gets a much, much closer look at being the starter last season.
Some players don't look great in practice but for whatever reason ball out in the games. Those type of players are the hardest for coaches to evaluate because it is in practice where players earn their playing time. I've run into the same problem with baseball. I had players over the years that simply weren't practice players and it took me half a season (and the starter struggling) before I figured out he should have been starting the entire time. It's not an easy decision for coaches when what they see in practice doesn't translate to the game, especially when it is a player who may not look great in practice but can ball out in games.