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SMTP needs this so desperately that it's worth making a customized solution - if there is a chance that making the standard generic would slow its adoption or development, it's probably best to do it this way. None of the others underpin, necessarily, a global communication infrastructure that powers the world of business.

There are a lot more protocols that would benefit from the more generic and much simpler solution of "define a standard port where the service listens over TLS".

IRCS has settled on 6697: https://tools.ietf.org/html/rfc7194

IMAPS uses 993.

Just having a dedicated port for TLS-wrapped version of the protocol wouldn't help here. HTTPS has 443, yet we still need HSTS to enforce it.

That said, dedicated SMTPS port would still be beneficial by cutting down at least 2 RTTs (HELO/EHLO + STARTTLS) from overall transaction time.

XMPP is easy to enforce SSL connections on...