And although you can say a lot of things about SESAR and wasting an atrocious amount of money, it's even difficult to blame the 8.33kHz debâcle on entirely on them.
A good short explanation is given
here
The main reason is a lack of willingness to cooperate at European Level and to keep each of the dust gathering telecom frequency managing departments, well shielded from their political masters by a smokescreen of radio-physics and geography.
My country is not without blame either : Our DefMin is even better than its French counterpart when it comes to frequency hoarding and bull****ting at technical meetings.
Clearly small GA-interests were largely ignored. It would actually make sense and be possible to keep the lower flight levels and departure / approach frequencies on 25 kHz spacing. But hey, you don't expect them to do an effort for "those stinkin' rich people and their aeroplanes".