Because prices go up exponentially. Wait till you check the 8A...
I used to think the same during my first Asp trip. I thought the price of the 6A is insane, and mounted a B, and was happy with it. I told myself, this is fast enough, I wouldn't even see the difference. See, the human brain has this need to justify having a cheaper piece of kit. But then the time came for the second trip, and I had a lot more money at my disposal. I figured, since I can resell the equipment without taking a loss, why not put all my justifications to the test? So in the name of science, I mounted a 6A and was even happier with it. Was it a night and day difference? Not really, it was noticeable but nothing too major. Then I got to a situation when my tank went through a significant drain and I needed to fill it up all the way to the top. Now that was quite a difference. Bottom line? I never took that 6A off the Asp. These days I'm flying an exploration 'conda, happily mounting a 7A. Why not? I can afford it, and I can always resell it at the same price I bought it for. With that system in place there is virtually no reason to not mount the best scoop you can afford.
The only reason I'd see taking a 7B as an option was testing landings on extremely high gravity environments - though, to be honest, I landed on that 9.7g world yesterday with my 5D thruster conda and still had my 7A scoop on, because I couldn't be bothered to replace it
Anyway, I respect dognosh's opinion, and also entirely disagree with it in this instance; especially since I've been there, thought those same thoughts, and came to realize that you mount a B simply if you can't afford the A. To be clear, this is from someone who does explore, I don't buckyball (I do skip some frozen balls if ice orbiting M class dwarfs - you see one, you've seen 300 billion..) I'd never empty my bank balance to the point of not having enough for a rebuy in order to get the A - but otherwise, the explorer's golden rule applies; best scoop you can afford in the largest internal you have. Hope that helps