Took me way longer then i care to admit to find this message on the mailing list:
https://www.mail-archive.com/kicad-developers@lists.launchpad.net/msg26837.html
In it Wayne said he is not really prepared to switch to something different then wx if there is no good reason for it. (Basically every framework has its problems and it is hard to predict if any other thing like Qt really is better on all platforms.)