I'd guess because there's more season ticket holders not on the League Cup scheme than are on it so most couldn't buy until today. City have handled this poorly imo. Should have just sold to people on the scheme last Monday (15k+ points) & Tuesday (anyone else on the LC scheme) then opened it up to people not on the scheme. No idea why they reduced it by just 1000 points per selling window. They don't help themselves sometimes.
I think another reason is that OSC branches got pretty well everything they asked for so a lot of people who would have normally bought tickets the normal way got them through branches for this one.