Lead times are also very important, it takes about a month to get something from china to the US. This means that you need higher inventory levels in order to be able satisfy the demand without considerable shortages.
When you take into account the variety of configurations available its very hard not to get low rotation items on your inventory. Specially if you are expected to always have stock and when your inventory levels have to consider that it takes a month to get an item to the US.
Shifting assembly to the US reduces the number of SKUs imported and allows an easier control over inventory levels and supply chain costs.