I think it's one of those things where if you've ever seen him play, or talked to a scout that has seen him play, those typical reservations just get thrown out the window.
The HR derby videos from two days ago are absolutely insane. He was hitting laser beams that less than 1% of professional hitters, at all levels, are capable of.
Also, calling him #1 has a lot to do with some of the other prospects in baseball that have graduated (ex., Bellinger, Benintendi, Swanson) or taken steps back this year (ex., Meadows). Of course you can still very much opine that Robles, Torres, Rosario, Moncada, Mejia, or Senzel are probably better at this moment in time. Rodgers and Jimenez in the top tier. Devers in that tier too. I think Kyle Tucker is top 10. Bo Bichette is the best prospect on the planet.
I guess the argument for calling him #1 so early is plainly that his hit/power combination is unrivaled in minor league baseball. This is a possible Miguel Cabrera. You can't say that about any other player in the minors right now.