Since nobody seems to have any concrete info and we're just making guesses, I might as well take a stab...
I'm much more a BSR guy than a Lodge guy so my speculation is based on BSR.
My main guess is it cost money to create new non-heat ring patterns so it was never a motivator.
Farther out there, maybe there was some belief that a heat ring went beyond fitting a wood stove eye, it was thought to have some other practical purpose, maybe helping "trap" heat under a skillet or something like that? Yeah, that's a stretch, but stay with me...
They switched to DISA automation in the late '60s. This required creating new patterns compatible with the DISA machines. So they left the heat rings on their traditional skillets with these new patterns, fine. But they also took advantage of DISA's production capabilities to introduce some new designs in the years afterwards, and that may give us some insight on their thought processes...
I have four examples of those - two sizes of cornbread skillet, a square skillet, and a chef skillet. The cornbread skillets, no heat ring. Makes sense - they're designed for the oven, not the stovetop, don't need heat rings. Then the square skillet, no heat ring. It may be designed for the stovetop, but really, how would you do a heat ring on a square skillet? It's be strange and impractical (and, as a side note, these examples, along with the chicken fryer, dispel the notion that BSR made "nothing but heat ring skillets"...)
But then there's the chef skillet, released in the mid-'70s. And it has a heat ring. Why? It's one thing not to change the design of their existing skillets when new patterns are created, that could be as likely lack of thought as a calculated decision. But the chef skillet was a new design, why include a heat ring? And don't give me a story about it being to accommodate the vestigial wood stove users by then, especially for a modern design made to be shaken and toss food.
No, there had to be a reason to include it when other new designs of the era did not have one. They thought it needed it, that it served some practical purpose beyond fitting in the eye of a wood stove, whatever that purpose may be.
Or it was something stupid like they thought it looked pretty. I don't know, I'm just making this up...
Happy Holidays everybody.