Marketing, I Think.....
This is conjecture, but I have an idea that Frigidaire's lack of a combo was based in marketing needs.
Remember that Frigidaire was out there with a wash action like no other on the market - one that required explanation to the consumer. The famous teddy-bear ad, showing the toy being wafted up and down in the wash, was just one of the tools GM used to put the concept over.
But a combo is - by necessity - a horizontal-axis machine. If I'm guessing right, someone at Frigidaire didn't want a second unusual wash action paired with Jet-Action. I think the fear would have been, "We don't need a reputation for making all kinds of strange machines."
GM was always careful to make Frigidaire a styling leader; I think that effort was very important to putting Jet-Action over. If the console was gorgeous enough, maybe Milady wouldn't think too hard about that unusual agitator that went up-and-down instead of side-to-side. I would go so far as to say that if any other manufacturer besides GM (or GE) had tried to put Jet-Action over, they would have gone belly-up; it took the reputation and marketing savvy of a really trusted company to pull it off.
But if Frigidaire had done combos, Milady might have thought, "They have that thing that goes up-and-down on some machines, and others don't have anything at all - they just go 'round and 'round. Nobody I know has anything like that. Maybe I should look at some other brands, too."
You may agree or not, but I think combos just complicated the sales message, and that was something GM was very, very good at avoiding at that time.