It's not difficult, but it's not as simple as installing the client and then just entering the server info. You have to use the little wizard to set up your sound and generate a certificate and register or whatever (been awhile since I set mine up), which is not intuitive and requires a little bit of explanation.
It's also irritating if you've did have it at one point but lost it and didn't back up the certificate for whatever reason, which I discovered after my drive bricked and I couldn't get back into my guild's server.
Again, I'm not saying it's hard to do -- it's not -- but even something as little as having to read a setup guide or run through a wizard is going to turn people off who are used to being able to jump right into using their voice chat clients after a five second software install. I can see why some guilds, especially those that run a lot of PuGs, may feel like it could be a roadblock for filling spots and would rather stick with what is most common on their server.
I'm not on a crusade to convert people to Mumble. It's my personal preference, but I don't feel like I need to be on a mission to force people to use it.