It's the same thing makes an H fare more valuable than a K fare: the fact that AA still offers it when the plane is closer to full.

The whole concept behind fare buckets is that airlines expect people to buy the cheapest tickets first, and when those tickets are gone, the next person who wants a ticket pays a higher price.

Since B is cheaper than Y, nobody will buy a Y ticket until there are no more B tickets available.

Also, in most cases B tickets have an advance purchase restriction (usually 48 hours), to force people who truly go at the last minute into the Y fare.