Simple, OP and some people just don’t know what they are talking about. There was no “aesthetic reason”.
One of the big changes in GNOME 40 (that would be 3.40) was the introduction of GTK4. People used to assume that the gnome major versioning scheme was tied to GTK, so loads of people were asking the devs when GNOME 4 was coming out.
To demistify this idea of one being tied to the other they just dropped the “3.”, specially since that part wasn’t that relevant and started with the 40.