SImple. because that's where it needs to be from the point of view of the Online part of BRM.

When you open up a Category, you see your inventory organised according to Product Family.

It is at this level that we show your prices.

You can see this clearly from the image below:

Products Lines represent sizes within a Product Family so it wouldn't make any sense to have prices that low down and Online wouldn't work.

If you find yourself needing more granular pricing you need to create a new Product Family.