Same reason why his father's choice of Souter shouldn't have been trusted and Reagan's choice of Kennedy shouldn't have been trusted and Ford's choice of Stevens shouldn't have been trusted and Nixon's choice of Blackmun shouldn't.....and all of those looked more trustworthy than this Miers.
If you had said O'Conner, I would agree with you. When Reagan nominated Anthony Kennedy to the high court, he garnered support from the largest pro-life organization in America, National Right to Life and other pro-life groups. After Kennedy was on the high court he began to reverse himself on the abortion issue. On issues related to crime, he remains a solid conservative, while somewhat moderately conservative on other issues. Don't forget, Kennedy was Reagan's third nominee, after Bork was shot down and Ginsberg withdrew for being a pot head.