We did that for several reasons--making it easy to filter by checker was one of them. Even though there are relationships between a defect's type, category, and impact, having them as separate filters makes it easier to deal with. It also allows you to filter on those attributes individually. The simplified filters are especially a benefit to new users who don't have experience with the 5.x Defect Type filter.
It is, and the 5.x Defect Type filter tried to do that, but it was quite complicated and still didn't convey those relationships clearly. For example, a single checker can find more than one type of defect, which led to checker names being repeated. To accurately filter by checker, you had to check ALL of its entries in the Defect Type filter, and not many people understood that. In the 6.0 design, you just check a single box for the checker you're interested in.