they didn't believe in the movie, doesn't mean they were afraid of the message.

A trailer costs next to nothing to make. There was no trailer because of the anti capitalist theme. Fox actively hid the movie to avoid jeopardising their corporate advertising revenue.

they could've easily just not mention it at all if it weren't important.

Exactly why your eugenics crusade is not mentioned.

yet they chose to say why.

Because it was presented in a comedic fashion. It was funny, without any Nazi overtones.

they chose dysgenics

They chose fertility. Not dysgenics. Not eugenics.

Or, you know, skip the first 2 minutes and just enjoy the comedy.