Why? it's widely deployed, actively maintained and more importantly reliable. 10+ year old support threads are mostly still valid advice.

It needs a bunch of milters to have basic functionality, such as DKIM, SPF and DMARC. OpenDKIM and OpenDMARC are not well-maintained, reliable or nice to use.

Postfix was designed years ago, but that doesn't do it bad, it is a beast, it can handle 5k mails per second (DKIM signed) with an small tunning in a modern hardware without problem.

Postfix doesn't have DKIM support though. You'd have to use unmaintained OpenDKIM, no Ed25519, bunch of known bugs. It's not good. 5k/s is also really mediocre on modern hardware.