Brussels is in Belgium

The tractor sounds like CKD, ie bolted together in Mexico from the parts supplied from the factory in Germany. I would have expected Deere to have all of the parts in the US for USDM products, however, the more parts they stock, the more it is ultimately going to cost you as they have $$$ tied up in parts collecting dust on a shelf somewhere. They might have had a run on these parts or the normal parts person might be off work, or made a mistake.

The part(s) could have been shipped over night at a cost, or they could have gone by boat. If they have indeed come from Brussels, not Mannheim, it's a 15 hour flight. Anything could have happened between where they started and your place. They could have been involved in a road traffic accident for instance, or if they came on a boat the container could have fallen off and be currently bobbing around in the north Atlantic. Also the parts would have started in Europe, because that's where the tractor would have been designed and aside from the Mexican versions, built too, because Europe is a massive market for tractors of that size. Once in a while people with US assembled machine have to wait for parts to come from the US.