with what the rest of the world uses, because they insist on custom non-metric sizes. Just like paper. There are many more Euro pallets in use than US-sized ones.
Though in this case, the US size may actually win in the long term, because standard containers are designed to accomodate US pallets optimally. The Euro variant does not fit as well. There is a slightly wider Euro container variant designed to play nicely with Euro pallets, but with ever increasing ocean-crossing container shipping, these are on the way out.
One problem of the smallest variant of the US pallet (35 × 45.5 inch Milspec, 40 × 48 standard type) is that is does not fit trough standard European doors (which are 850 mm - Euro pallets are 800 mm, US mil pallets are 889 mm on the smallest side, standard type even larger).