I never read the heavy or technically convoluted explanations of how ms office 2010 ended outlook support for divs.
But here's where I think support ended from a more business perspective:
An email is essentially just a "push" message; the same inheritance of SMS texts or social posts. So, essentially it's fundamentally core to the act of communication. During this evolving phase, implementors have attempted to inject more sophistication into the art. One way is that they attempted to just push entire web sites into your email, etc.
That's good or bad, depending on potentially nefarious intentions, etc.
Anyway, Microsoft wanted a part of the action. Their claim to fame is the OS + Office. So, if the future of communication is via [divs] or html standards; well they can't have it. Microsoft's goal is to allow it's .doc files to be easily converted into email content. [thus, they removed css-positioning from their outlook msg clients]
So, how does this explain Gmail's lack of support?
Gmail also had it's complement of gdoc ecosystems that would be negated if the sophistication can be done elsewhere. (aka the plethora of html txt editors) - & they become just another delivery agent.