I'm not the authority but to be brief I'll give my opinion as if it were fact while admitting up front that its my opinion at the moment.
Its a good question, and the religious reasons are that this will benefit posterity, the next generation or two of children -- maybe many more. The secular, more immediate reason, is that complex economic ties between the two huge nations is a discouragement to war. The USA's immediate motivation is to promote peace in the immediate or near future as well as economic stability in the world (foremost for its own benefit). The immediacy places it in the secular realm. "Lets avoid world war at all costs," followed by the religious "won't it be great to have warm relations between us and the Chinese? Imagine the shear potential 50 years from now!"
Religion deals with the idea of posterity, putting off the immediate for some future or dream

. (Social Security is a 'Religious' plan in this sense, whose immediate secular purpose was to ease retirement of seniors but whose religious purpose was to make retirement for all a reality.) Religiously, we are giving up certain immediate economic advantages, but secular-ly we are stabilizing the sphere. I know this discounts all immediate concern as non-religious and classifies previously religious material as secular, but I've got to draw a line somewhere. Now is secular, and the future is religious. And that's why we want good relations with China.