hey guy, there's a plenty factors that played a role : economics, big/slow use, effective,... By example the DCU is a common camouflage in some contries, not only USA and Netherland army. There's also some africans and arabs countries that wear it.
It costs some money to develop a new camo so it's sometime better to buy/use the one of another country. Moreover, when you've a contry that's not often in desert environment, like Netherland army, it has no sense to make his own camo and it costs too many money. It can be one of the reason.
In US army, there's a multitude of desert camo : DCU, ACU, marpat desert,... some differents camo for some different environments : woodland, desert, night, snow, digital, urban,... by example, in Belgium we only have two differents camos at this time. And never forget that USA spent some money for their uniforms, not as Engliand or other european countries. In our european countries, we keep a camo for decades. Some of our uniforms are obsolete and inadapted for some combat environments. We developped them one time for one type of environnment, but never changed them after. As you can see now with multicam, US army found that DCU and ACU were "ineffective" and so changed them for a multi-environment camo : multicam.
And you can add culture and tradition factors as Ladrang said.
And there's still some difference with the american DCU. Both designs differ. By example : epaulets on Dutch army DCU uniforms !