It costs money to validate a new processor on an old motherboard, and no corp wants to waste money on a product they have already sold.
Turns out it's really hard to guarantee compatibility with several years of third-party products which weren't designed with your new fancy features in mind.
The flash space was pretty forseeable and they dealt with it, more of an excuse than anything.
I'd bet customer perception is also a factor; there's a risk the old boards (not even made by Intel) die and cause problems, and Intel wouldn't want to deal with press/comments like "This CPU stopped working after 2 months" or "I installed this new CPU, and within 2 months it killed my motherboard that had been working fine for 7 years".
They've released CPU upgrades with the same socket before, I'm sure they have the sales data to know how that performs vs new socket.
Laptops have outsold desktops for well over a decade, and their CPUs pretty much non-upgradable. I can't easily find a nice chart to reference, but intuition tells me the desktop industry is similarly trending towards complete "system" sales vs individual parts. In other words: Most people don't upgrade their CPU, they upgrade by replacing the entire system. If true, this also means the socket would be almost entirely irrelevant to sales performance.