Not everybody's working on web or something running untrusted code. Not fair to force people to pay a financial or performance hit for something they don't need; the world loses if peoples' scientific computations take 20% longer to run because of some ugly patch to an obscure vulnerability in speculative execution, or because the university can afford less computing resources because the only option is expensive ram.
If ECC was made for the same markets as normal memory, it would cost <12% more and make entire computers cost <2% more, with no meaningful performance hit. It's reasonable and should be mainstream.