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There were several issues:
Zenith saw that their idea was not winning out. Although porthole operation gives a larger image, there were sufficient cases of complaints about cutting off material that sets were then made with a switch to go to "full picture" mode instead of "full circle."
There were also problems in color tubes with scattered electrons from the bell of the tube fogging the picture when the corners of the picture were bright - porthole operation would be worse.
There would also be X-ray considerations, and the special design of the horizontal sweep and high voltage, especially converngence if the size switch was required.
Professional monitors had an underscan setting to show the edges of the scanned picture, and this resulted in double sets of convergence controls.
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