Sendmail wasn't really the first per se, but it was the first that worked well. In the early 80s there were a lot of competing email protocols and systems and implementations, and Sendmail could talk to all of them, in a "bug-compatible" way. This is also why sendmail.cf is so bewilderingly flexible and complex.

Later email systems like Postfix and qmail in the 90s had it much easier as they operated in a much simpler ecosystem.