There is something about the simplicity of singing along with the ukulele in an overproduced world. Its also a little silly, so you don't have as much pressure to be a virtuoso (though there are plenty). Its an instrument that is at the service of the song. And its usually a simple, pretty song, somewhere between tin pan alley and hall and oates. Unless its a Bow Wow cover.
I just think that its fascinating that this is what post modernism sounds like.