Wireless is inherently more prone to this type of attack because you can listen to it, and if you can listen to it you can try to crack the encryption. With wired connections (we're not only talking about networks here, wireless keyboards too for example) most of the time this is impossible, even if you can somehow get at the wire the chances that no one is going to notice are non-zero. With wireless they can be exactly zero.
In practice not everything that connects wirelessly uses encryption, not everything that uses encryption uses unbreakable encryption, not everything that uses breakable encryption can be retrofitted to use something better - especially if it's a standard - and not everything that uses unbreakable encryption has non-vulnerable users (ie. users ignoring SSL warnings caused by a MITM WiFi hotspot, which has been done, though I don't have the links on hand).