I know I'm oversimplifying things, but if progressives feel their only choices are a "mean, hateful" God, or secularism, then they'll go with secularism. Since conservative Christianity has dominated Christian perspectives on religion, progressives see no point in associating themselves with it
I don't think this is the main reason why progressives are less religious, though. Sure, there are those who are turned off by religion because they associate it with the religious right. But I think the main factor is the fact that progressivism, as it has always been but probably more so than ever, is correlated with a rationalist/secular viewpoint. This is not to say that religious progressives are not "true" progressives, of course.
