During the regular season, ABC is pathetically weak at 10pm virtually every night. If this had been anything but a filthy little game show, ABC brass would be doing backflips at those numbers opposite a heavily-hyped episode of a popular series.
Right now, it's not about getting the best rating ABC can get for Millionaire. Obviously, Millionaire would pull better numbers earlier at night. Right now it's about getting some kind of number at the 10pm hour, and Millionaire takes care of that for them better than their regular shows. I'm not sure if some of their 10pm slots currently even have regular shows.