The EZE flight stops off at GRU so in reality you have two flights in one.....I've no idea how BA work out the number of redemption seats available to each city but my guess would be that the GRU stop does have a big effect on the number of redemption seats available from LHR-EZE. The quicker they ditch the GRU stop the better.

Of course Jan-Mar is the peak period for travel down to here so it's never going to be easy anyway. It's even quite hard to get seats with IB.