What's exciting to me is, both the first interstellar object we've observed visiting our solar system (Oumuamua) and the first Kuiper Belt Object we've visited have been "unusual". That strongly suggests to me that the unusual is more usual than we would have suspected, which means there's probably a great deal of opportunity to increase our understanding of the universe!
Oumuamua was literally the only extra-solar object known. There was nothing to be picked.
Similarly for Ultima Thule. They may have been able to find a different target, but it was very difficult to find any target at all in the first place (lots of Kuiper belt objects are known; the difficulty was finding one that could be reached by New Horizons). See this Twitter thread that was linked elsewhere: https://twitter.com/Alex_Parker/status/1077986070128668674
At least in the case of Oumuamua though, being non-spheroid wasn't really the weird thing about it. It was that it exhibited comet-like acceleration, without any visible off-gassing, and without breaking up as it passed the sun, as a comet would be expected to do. Also the fact that from our current understanding, it is much more likely for a comet to be ejected from a solar system than an asteroid, so it's surprising (not impossible, certainly, but unlikely) that the first interstellar object was more like an asteroid.
Pluto may be a KBO also. Further, it's possible some moons of the gas-giant planets are captured KBO's.
The unusual thing about Ultima Thule is that it's in a nearly circular and "flat" orbit, which many experts interpret to mean it's mostly undisturbed from its point of origin. It's a prime "fossil".
Pluto has a "disturbed" orbit such that its origin is currently unknown. Same for gas-giant moons. Pluto has also been turned inside-out, perhaps multiple times, by a still unidentified force. Ultima Thule is probably mostly as-is since formation.
> Pluto has also been turned inside-out, perhaps multiple times, by a still unidentified force.
Whoa, come again? How do we know this? What does it mean for a planet-like object to be turned inside out?
Inside out just means that the local equivalent of Earth's geologic plates seem to be subducting below the surface and fresh geologic plate equivalents are abducting back up. You can estimate how long a planetary surface has been exposed to the elements by the statistical distribution of number and size of craters. We have a reasonable idea of the likelihood of objects impacting Pluto, and it should have a lot of craters. Pluto's moon Charon has many more visible craters but Pluto has large areas with no large craters, so some sort of geologic process must be going on that's recycling areas of the landscape and covering or subducting the craters.
Given lack of fast erosion from liquid water or significant wind (in a very thin atmosphere), whatever geologic process is smoothing out Pluto is happening quickly compared to other points of reference we have.
I use the example of Earth's geologic plates, but that's only one possible explanation. There's also speculation about freeze/thaw cycles of the planetary material and atmosphere (since it has an irregular orbit), movement of water-ice mountains, cryovolcanos, and other theories.
