Because the G-20 wants to remain the G-20 and not the G-19; you can't kick out a member every time you don't agree with their opinion, regardless of who wrong you may feel it is.
Aren't these types of alliances fluid and opportunistic?
I personally agree, I don't think we should give a rat ass what a regime like SA thinks. Part of the point of the GX/UN/WTO/<insert bureaucratic partnership organization here> is to slowly co-opt authoritarian regimes into a pro-market/business mindset with the belief that democratic reforms will slowly follow. For a quick shallow example (this is technically off-topic): China, for awhile looked like it was on a path to give it's citizens real agency in their political future, some attributed this to their embracing of a limited form of capitalism, but now that the economy is contracting, their freedoms are being reproached... The same logic was used in the lifting of sanctions against Iran, which hasn't stopped them from hanging gays from building cranes yet, but the hope is long-term it'll force the hardliners out of power.