The reason it went number one is because it was a new album from Radiohead. Also people wouldn't have given it a real shot after the official release date?
Actually Kid A was the first album that made almost a genre change, which is why people considered it 'awful' in the first place: they were expecting the same old Radiohead and they got something completely different. So yes, had they released an album similar to the previous three, I would agree with you, but what they did was quiet risky and the free downloads allowed people to give it a chance.