So to add a bit...
Let's look past availability which is an important determinant in its own right.
This is principally about the physical layout of the specific room - meaning how much space is there around the seating area
The way this stuff is designed, the 5.1 speakers are to the left and right of the audience about 2/3rds - 3/4s of the way back, firing across the audience (L to R, R to L)
The 7.1 pair are behind the audience firing forward towards the screen.
The more discrete sources you have, the smoother the motion of sound from one speaker to another, the more precise the placement and the greater the illusion it creates.
In many rooms, including mine (alack, alas) there is no room to fire across so the 5.1s become the rears. A dipole design helps take the edge off it but still it simply cannot create the same depth of illusion.
Let's look past availability which is an important determinant in its own right.
This is principally about the physical layout of the specific room - meaning how much space is there around the seating area
The way this stuff is designed, the 5.1 speakers are to the left and right of the audience about 2/3rds - 3/4s of the way back, firing across the audience (L to R, R to L)
The 7.1 pair are behind the audience firing forward towards the screen.
The more discrete sources you have, the smoother the motion of sound from one speaker to another, the more precise the placement and the greater the illusion it creates.
In many rooms, including mine (alack, alas) there is no room to fire across so the 5.1s become the rears. A dipole design helps take the edge off it but still it simply cannot create the same depth of illusion.