Largely because he repeatedly pandered to them on the issue of abortion, plus they've by-and-large have bought into secular right-wing politics.
The reality is on abortion, according to Chief Justice John Roberts, who is a "pro-Life" Catholic, Roe v Wade is not likely to ever be overturned because he says it's the "law of the land" for many decades now, plus the majority of Americans simply do not want it overturned in its entirely.
Even within the "Pro-Life" camp there's no agreement, such as what about incest? rape? If abortion is "murder", and an 18 year old girl gets one, do we try her for murder and give her the death sentence or life in prison if she's convicted?
And if it were to be left up to the states, then the only groups it would affect would be young girls and/or lower income women because middle and upper-income women would be able to get an abortion in another state.
BTW, I am very much pro-life on this and other issues, but I simply do not feel that the federal nor state government should determine if a medical procedure, or the denial of one, should be mandated by the government. Talk about "government overreach", which most conservatives say they don't want.