Sendmail has a terrible reputation for security. While the latest releases are very good, past releases have been less than secure and that is where the reputation has come from.
Alternatives, like Postfix and qmail are proving to be much more popular, and have better history on security. All of this has led to Sendmail being removed from NetBSD.
Sendmail wont be unavailable on NetBSD, it's just not being included as part of the standard package - you can still incorporate it later if you want.
Is this fair?