Why didn't Microsoft push for this instead of TPM 2?
That's like saying why doesn't the government focus on creating regulations for meat rather than funding a cure to ear infections. The two have nothing at all in common with each other. Not in use, practice, or application.
Add to that, ECC has downsides. It's more expensive and it's slower. It has its place, but that place just simply isn't on most desktops. If you're running a computer crunching critical financial transactions, yeah ECC is a good idea. If you spend your time teabagging other players in CoD then the impact of a RAM error will very likely go completely unnoticed. What will be noticed is that ECC tops out at 7200MHz at a significantly higher CAS latency than the comparable non-ECC memory which is 8800MHz at a lower price.
On the flip side TPM 2.0 provides some meaningful security benefits. You may not care, but many people do, especially laptop users with full disk encryption.