It was partially because of the lack of economic incentive to go east: Columbus went west trying to find an oceanic route to the far east to engage in profitable trade: the same didn't apply to China. The other factor is the width of the Pacific: it is about twice as far to get from coastal China to the Americas of the US as it is to get from Europe to Cuba. Thus making it both much much more difficult to reach and any potential journeys much more unprofitable.