For the rectangular look.
On some of those old sets it was hard enough to get enough vertical deflection, let alone enough to give it a porthole look. Remember, sets had to be affordable to operate and repair.
Help me out on this one, guys.
I know many FBPs had lettering and/or numbering on the bottom portion of the screen, covered by the mask, that would have shown in a porthole. I can't remember if the bonded ones had lettering/numbering or not.
Masking the sides of roundies would have further reduced the viewable area. But...you didn't have to worry about those pesky corners when replacing a roundie.
Phil