There’s a very good reason – it wasn’t discovered until 1992. At that stage, Voyager 1 was almost all the way across the the Kuiper Belt and Voyager 2 halfway through it!
From the article –
“But, perhaps most important is the question: could Voyager have flown by a small KBO as New Horizons will do this December and January? Regrettably, the answer is no, for a number of reasons. First, even once the Kuiper Belt had been detected in 1992, the Hubble Space Telescope (the only telescope capable of finding such distant flyby targets, even today) hadn’t been repaired to properly focus light. That repair didn’t occur until December 1993. By then, Voyager 1 was exiting the Kuiper Belt near 55 AU, and Voyager 2 was near 42 AU. But even after its repair, the Hubble wasn’t sensitive enough to detect KBOs as small and common as MU69, so there would have been no way to find a flyby target—that capability only came in 2009, when a more advanced and sensitive wide-field camera was placed aboard the Hubble during a servicing mission.”
New Horizons is the fifth spacecraft to traverse the Kuiper Belt, but the first to conduct a scientific study of this mysterious region beyond Neptune. Credit: NASA/Johns Hopkins University Applied Physics Laboratory/Magda Saina