Earlier quoted context omitted.
Interesting. This might be explained by the switch from old growth to new growth^. Older trees would be larger, and have more core to work with (I think knots are mostly from the outer layers of the tree). ^ this is pure speculation
The core (heartwood) is the worst part of the tree. Now the trees are harvested so young that the entire things are junk heartwood.
For hardwoods, it's typically the heartwood that's more desirable. Walnut and cherry both have pretty strongly contrasting sapwood that does not match the customer's expectation for the color of those woods, for instance.