I suppose it's mostly from a technical perspective: Single code-base, single binary, written in modern language (Go).

There are advantages to an integrated solution: The junk/nonjunk flags you set in your mail client are used to assess incoming email (based on full sender/domain/orgdomain/spf/dkim/ip/ipsubnets). I'm not sure how common/feasible that is with solutions that combine mail server components. I've used postfix & dovecot before mox, I admittedly didn't closely look into it but assumed that combo cannot do that.