Because, in Europe, above FL095 (which is where most airway routes will be), you need BRNAV, and the only way this can be met practically in the GA context is an IFR GPS.
You can meet it legally with a KNS80 and antenna filters for FM immunity, but it won't be much good for real because airways ATC treat VORs as waypoints and will happily give you a DCT to a VOR 300nm away.