As far as I know, the DANE spec (RFC 7671) requires DNSSEC to be enabled, while MTA-STS does not.

MTA-STS was standardized explicitly to support the (nearly universal) use case of mail providers without DNSSEC. Even O365, which ostensibly supports DANE/DNSSEC for email security, does so only for select customers and not for ordinary ones (go look for the TLSAs).