Have no idea. I'm assuming they do this to take the temp of a certain swath of the tastemakers who are engaged with fanboy films (heroic Hollywood, the wrap, collider etc). Then use that as a gauge as to whether to move up the non fanboy media embargo date.
The fact that many of the reactions are noting issues with the movie leads me to believe some mainstream critics will not like it. But if the movie is as fun/funny (not serious) as it seems, it'll get a soft-pass from many as many flawed but fun Marvel films have in the past.