In the Bavli, you have many times a question that was brought up in one generation, and then answered (with a source from a baraisa, for example) only generations later, such as in the times of Ravina and Rav Ashi. (There's even a rare example, in Sanhedrin 43a according to the girsa of Rabbeinu Chananel, where a question is answered by Rav Ravai of Rov, one of the Rabbanan Savorai.) Only if it wasn't resolved by then will it be labeled תיקו.
In other words, this term is part of the editorial apparatus of the Bavli. Well, the Yerushalmi didn't go through that kind of editing, so a question left open was left that way - a future generation of Amoraim, had there been one in Eretz Yisrael, could have found an answer, but there were none.