Maybe Minako's trying to keep them apart for now so Usagi can spend her time training or whatever. It's clear that Minako is seeing the whole picture, or more of it than anybody else is, and to me, it seems like every move she makes is deliberate.
Zoicite guessed Endymion pretty quickly so maybe by keeping Usagi away from him, Minako is again taking the "princess" thing onto herself.
By not associating with Tuxedo Kamen, Usagi is safe from being suspected as the princess, perhaps?