I would guess the thought process is that a July 9 release date will give more time for people to get vaccinated and for theaters to open up more, making Black Widow more of a sure thing.
Most of what Marvel has coming up in Phase Four of the MCU are unknowns.
However, it’ll also be competing with Top Gun: Maverick (out the weekend before), so I’d argue that’s a factor as well.
People don’t know The Eternals and even though Kumail Nanjiani getting ripped has received positive reactions, a lot of people could not tell you much about that property as a whole.