It's supposed to be lighter for purely local installations with an single binary, and was the default mta for Debian for a long time for mostly that reason. some of the points made doing that decision is available here https://wiki.debian.org/Debate/DefaultMTA#Comparison

For the most part postfix/dovecot have become the default for full scale mail servers but back when the Debian defaults were decided there was still some uncertainty about what would replace sendmailNG as the lightweight default mta.