Apple focused on monitor output, Apple and Commodore went with TV output. I think this is key to why Atari (and Commodore) did not make 80-columns standard. 80-columns is not great on TVs. Yes there is XEP-80 and the Commodore 128. Both are flawed, both not a standard on each platform and both were too late.
And yes you could get a monitor for both Atari and Commodore computers but by making 80-columns standard on their computers, it would have angered their costumers who used TVs with their computers.