However, they have not proven to be popular outside of gliders. There are very few examples of them, with only the Europa XS, the B-52, and the U-2 as modern(-ish) examples I can think of. (And even the Europa XS sales have now shifted largely over to tricycle gear.)
Below is a monowheel variant of the Europa XS:
While it can be argued that maybe the relative absence of monowheels is because pilots are a conservative sort and will only fly things that "look right", that's a less compelling argument when someone says "your job is on the line". If the monowheel were better for a broad class of planes, the logical conclusion is that there would be commercial pressures which would lead to their broader introduction.
(This would have been especially true in wartime 1940s when rubber was notoriously hard to come by and minimizing the number of tires would have had instant payoff.)
Perhaps the technology to enable monowheels' theoretical superiority wasn't (and still isn't?) present? With the exception of gliders, outriggers are necessary, and those would be dramatically smaller and less draggy with modern composites as opposed to, say, chunks of wood or steel.
Anyone have any hard data on this? Surely NASA, or NACA, or some early airplane manufacturer must have dabbled in this and written down their findings...