Listening to his introductory press conference, it seems that the length of the contract was the most important thing to him and his wife. They both talked about starting a family soon and wanting a place where they can raise their kids. When he talked about his contract, Harper said, "this will take me to age 39" in a way that seemed to indicate that he doesn't see himself playing much past that age. They're both Mormon, and a beautiful new temple just opened up here a couple of years ago, and that might have played into it as well. But I bet if the Nats had gone to 13 years, he would have stayed there, because between the full no trade clause and the lack of opt outs in his contract, it's clear he and his wife just want to be in one place for the rest of his career, and that's exactly what Philly offered.