[old_tv_nut]

Cutting off the top and bottom probably made convergence easier, but I think it was mainly that porthole designs were long dead, and black and white sets had already gone from cut-off to rectangular - so going to flat top and bottom was only one stylistic step backwards. The RCA 15 inch tube had an internal bezel defining this shape. Thsi disappeared when tube construction was simplified for the 21-inch, but the external bezel was cutoff the same way.