@EvilMariobot A lot of it is down to licensing rights. The publisher often only had the rights for their own country whether it was down to costs since global is more expensive, licensor restrictions, on platforms that just didn't exist in the US (JP - PC-88, MSX, Sharp X1, UK/EUR - Spectrum, Amstrad) or just don't have the facilities. In this case Tokuma Shoten never published a game outside of Japan and people really only known them for being involved with the Studio Ghibli movies, even then the likes of Streamline, Miramax and Disney were bringing those out in the US. Also back then Japan really did like Hollywood movies.

This would have meant that another publisher in the US would have brought out the game but due to the 5 game limit on the NES, they had to be very picky and think of the cost. The latter was probably why Konami couldn't bring out the first Goonies game and probably their Ultra Games allotment that they did was also full (Konami were really prolific back then doing 3 platforms - arcade, NES and MSX). If it was a choice between publishing Labyrinth or an original game that didn't need to pay licensing fees, they would choose the original game unless they know that it'll be a profitable hit. Sometimes it's a free for all like with Rambo with Pack in Video, Acclaim, Sega and Ocean all having rights to bring out games based on First Blood Part 2 (Sega, Ocean and Taito also had Rambo 3).

Since Labyrinth didn't do well in the US box office, it would have put off any releases of the game and the NES was a mess distribution wise in Europe otherwise it would have got localised (Labyrinth did really well in the UK but the Spectrum game where it was the most popular computer at the time was cancelled...). Chances are that Activision may have considered since they published the C64 and Apple II game but we don't know.

@Ton: Because Japan-only?

My question is, why are so many games based on 80's American movies Japan-only (Goonies 1, this game)? You'd think importing them to their IP's native country would be a no-brainer...

(So help me, if I learn there's a JPN-only Princess Bride videogame...)

How did I not know this game exists?!