When will it get through? Ratings = Popular. Ratings do not mean 'best'. Popularity does not determine how good something is, it just determines how many people are watching.
Unlike BB7, BB5 doesn't need a bunch of excuses like "Oh it was against the World Cup etc". If BB5 wanted an excuse it could be that BB4 was a failure and many people didn't return to Big Brother after that, but it doesn't matter, I'm not bothered about that because regardless of how many watched it was still an entertaining series from start to finish.
I think if you asked the same question on a much bigger forum like Digital Spy most would choose BB3 and BB5 over BB7. Hmm but that forum has a huge bias in the favour of Aisleyne so it may be a little dodgy actually.