What an oxymoron: orla is a mitzvah tied to the Land of Israel, so how could it apply abroad?!
The Mishna (Kidushin 1:9) states that orla is an exception: "All mitzvot tied to the Land of Israel apply only in the Land ... except for orla and kilei hakerem [which also apply abroad]."
The Gemara (Kidushin 39a) goes so far as to state:
"Whoever says that orla does not apply outside the Land of Israel will have neither child nor grandchild "who casts line for the lot in the congregation of Hashem" (Micha 2:5) ... orla outside the Land of Israel is a precept transmitted to Moshe at Sinai."
Orla in the Land of Israel is a biblical prohibition, while abroad it is a halacha leMoshe miSinai (closer to de'oraita severity than derabanan). Both Rambam and Shulchan Aruch rule accordingly.
However, there is a major difference inside and outside the Land of Israel when it comes to safek orla: if orla status is in doubt outside of Israel, it is permitted (Mishna Orla 3:9, also Shulchan Aruch §294:9).
Abroad, as long as I don't actually see the fruit being harvested from an orla tree, I may eat it. While I may not eat fruit from my tree during its orla years, outside Israel if someone gives me fruit from their tree—even if it might be orla—it is still permissible to eat. And if I buy fruit from a store, I can eat it even if I know that the vast majority of fruit is orla.
There aren't many Jewish farmers living outside Israel, and many Jews live in apartment buildings and don't have gardens with trees. This is why, I believe, most observant Jews are not aware of the prohibition, although it definitely applies abroad.
