Redhat, and probably others, like to compile lots of packages with support for everything including the kitchen sink, so with sendmail you has hesoid, sasl and ldap support build in. This means RPM requires you to install these packages.
This makes it hell to instal *minimised* base systems for servers. The dependencies for sendmail require sasl, LDAP, and scads of other stuff that the server is not going to use and therefore should *not* be installed.
Of course you can build your own RPM without those dependencies but that takes quite a lot of doing. Updates are also a problem because you have to regenerate your patch that changes the package compilation options to saner values, and update your spec file, every time and update comes out.
(One could argue this problem is much worse on windows but that is a seperate flame war).