Two of the main UX reasons actually go against the use of subdirectories
I once fought a long battle with regional site managers on this topic. I've asked a related question on this site > How does a CCTLD affect user behavior?
National pride /comfort with the familiar
The idea is that, particularly for certain countries (France was a big issue) shoppers prefer to see a local CCTLD over a .com, and that this can have an affect on search page click through and conversion.
This advice is now 7 years old, but here's Google's Matt Cutts explaining why to use CCTLDs https://www.youtube.com/watch?v=GyWx31GeQWY
The complexities language and redirection
The idea here is that it's tricky to always get the right content in front of the right user. Decisions have to be made about how and when to redirect users and to what content, and in this process there is room for error and disgruntlement. There may be more clicking and thinking involved with moving around between separate linked CCTLDs, but the user is more in control of their own fate.
The Positive UX of subdirectory approach
Properly executed, users have a seamless internationalized experience without ever having to think about it, but beyond that, the experience is affected little by what's happening in the address bar. End users care about performance (speed) first, and content quality second.
In my case, I never needed to rationalize the postive experience side of things because the technical benefits were so obvious and strong, and no one could ever provide anything more than anecdotal evidence against doing it.