Well, I'd not be expecting people who normally come into London on a Victoria train line* to suddenly be coming into Waterloo instead? Wouldn't meeting inside Waterloo station add quite a detour & delay for them?
No.
To get to the South Bank from Victoria it's quite sensible to ride over Westminster Bridge, and from the south of Westminster Bridge it's as sensible to go via Waterloo (turn left past County Hall and right at the RFH) as it is to ride straight to the meeting point.
Alternatively, almost all trains to Victoria stop at Cla'am junction, from where it's a simple change to Westminster.
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I suppose what I'm saying is that Victoria has been convenient because for a smallish but influential minority it's been the obvious terminus. When we met at the South Bank before there were a good dozen of us who congregated at Waterloo.