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The development of the first color CRT started in the early 50's when round monochrome tubes were still being put in sets. They chose a round tube early in development (the envelope of the 15GP22s developmental predecessor was basically a 16GP4).
Zenith demonstrated a 20" rectangular 15G based color CRT in 54' and Westinghouse had a small production of rectangular color sets in 1958(IIRC) so they could have been available earlier. My guess it that because of poor color TV sales prior to the mid 60's it was not worth the money to develop a rectangular CRT with comparable production economy of scale to the round tubes until then.
BTW IIRC broadcast color monitors had an under-scan mode to observe the edges and corners of the picture.
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