No motive was ever given, as far as I know. I sort of fanwanked it as they were being led their by God, as it was some fulfillment of their destinies. Also, the thirteenth tribe were there, and perhaps there would be some sort of new start (New Caprica was an old start, after all).

Honestly, I'd always thought the Cylons wanted Earth way more than the fleet, if only to settle there as some sort of Promised Land.