They focused on those b/c the only thing that China would trade for was precious metals. Spain's trade was focused on silks and spices b/c they were high value items. They needed the currency for that. Acapulco came to exists basically b/c it was the staging area for sending silver to Manila and receiving spices and silks that would then be taken by caravan to Veracruz and shipped back to Spain.

Edit: I linked to it in another answer, but the wikipedia article on the Manila galleon trade has a good explanation of what the Spanish Crown was using the precious metals for: https://en.wikipedia.org/wiki/Manila_galleon

Charles Mann's 1493 also goes into detail about this trade.