I was told on the MTA-STS working group list that well-known URIs were chosen because the working group did not think email server administrators were smart enough to use DNSSEC.
I also asked why this is better than just standardizing on a port and using SMTP over TLS with existing certificate infrastructure, but they did not answer that question.
I also asked why this is better than just standardizing on a port and using SMTP over TLS with existing certificate infrastructure, but they did not answer that question.