Romney ran on conservative principles way back in 1994 when he ran for the Senate
A campaign flyer unearthed from the 1994 race lists a side-by-side comparison of positions between Romney and Kennedy for 24 election issues. A closeup of the flyer with the candidate comparison on the issues is shown below:
Romney held the solid conservative position for 23 of the 24 issues listed; the only exception being the pledge to maintain the status quo in Massachusetts regarding a woman's right to choose. A pro-choice position in Massachusetts in 1994 was a socially moderate stance accommodating the large majority opinion of voters in the state.
It's understandable how a first-time candidate in 1994 and former businessman running a crusade for fiscal conservatism with solid conservative positions on crime, welfare, foreign policy, healthcare, and congressional reform might accept the status quo on a social issue respecting the liberal constituency he would represent.