I know I've dealt with this question before—in my second entry here, no less—but I think it's one that I could stand to re-visit every once in awhile.
The most obvious reason is that Christo-Paganism, as I practice it, anyway, is partially based on a framework that makes sense to me at a very deep level because even though my immediate family wasn't (and still isn't) particularly religious, I grew up with a certain consciousness of Christianity because I spent fourteen years (kindergarten, plus grades 1-12 and OAC) in Roman Catholic schools, where prayer—whether or not you want to involve yourself in it—is a fact of life, and where involvement in worship of one kind or another was inevitable. And even though I'd actually rejected Catholicism by the time I was in my second year of high school, I always sensed something behind it (if that makes any sense) that I could get along with. My occasional brushes with other Christian denominations over the years—especially Anglicanism—gave me the ability, if not always the will, to see that not every church has the problems that I eventually grew to be unable to tolerate in the denomination of my baptism.
I became Pagan primarily because although I didn't think that Paganism of any stripe was perfect either, there are several things that Christianity largely fails to address, or address adequately, that many Pagans have made a central point of their beliefs and their actions. Things like caring for the planet we live on. Like women's equality and the unjustness of discrimination against people who are "othered" for any reason. Like the importance of our sexuality, whatever form it takes, to us and to the many ways that we experience the world. Like the importance of working for the betterment of the world, not because it could save us from eternal punishment but because it's the right thing to do.
I blend the two, though not always very well, because both approaches have enriched my life and strengthened my sense of compassion in their different ways. Except for a brief time when I was feeling the most embittered by my disillusionment with my original faith, I have always regarded Jesus as a great teacher. The love and compassion that make up most of his teachings are still the basis of my moral compass. And Paganism has taught me, and continually reminds me, that as much as we try to ignore or deny it, we really are all connected to each other in one way or another—and that taking care of each other, and of the planet we live on, is of paramount importance.
It isn't an approach that could work for everybody, of course; I don't think it's possible to have any such thing. Not with the huge variety of experiences that humans tend to have with the world. But even with its problems (which are, to be fair, mostly caused by the Christian part, especially the exclusivity that I've rejected but that's still such a big part of the mainstream Christian worldview), it works for me. And I figure that as long as it's not leading me to cause harm to anyone else, that's as much as I need it to do.