I asked a cloudflare engineer this and the answer was a bit vague but amounted to the failure rate being something like 0.5% which was too high for the amount of TLS sessions being initiated all the time.

Although I always thought it would be a nice feature for security conscious folks to be able to ennable. Or go ahead and use it on more sensitive sites only, e.g. banks.

Browser vendors weren't interested. They believed the additional latency of DNS lookups would impact customer conversion rates. DANE is catching on in the email world.

Because (to a first approximation) nobody uses DNSSEC, which is required for DANE to work. The "why nots" of DNSSEC are many and varied, from reliability to low utility to vendor support.