1. Not all of them are better than traditional designs: Many companies don't get rid of air gaps, ridges, or too high of excursion on the midwoofer to be better than a traditional design in every way.
2. Lies (often in ignorance to be fair) and archaic knowledge: Many still believe IMD is an issue with the waveguide moving too much, which can be true on a two way, but is transparent on a three way properly crossed over and designed not to have this issue.
3. Believing that nobody cares: Many people argue that vertical dispersion (which is almost always superior in a coaxial driver) doesn't matter, because they are so far away from the speakers that they never encounter issues with lobes making their speakers sound busted, or the reflections+direct sound balances things out, or they never sit on the floor, stand up, or have chairs at different heights.
Genelec has a whitepaper that covers issues like diffraction and mentions IMD, and I don't think they've ever made a non-3 way coaxial:
Emailing Genelec asking about disadvantages, the only thing that seems to be true anymore is that you can't get a massive magnet for a larger high SPL tweeter to fit, so for main monitors it's not as useful.