Fanaelialae
Legend
This is actually incorrect. The only really big selling items in the line are the core rulebooks. There's a very noticable decline in sales as they proceed through the line, and since the later books tend to cost no less to produce than the early ones, they're locked into a cycle of diminishing returns.
That indicates that they should go for really big, meaty Core Rulebooks, as they can set the price to give a good profit margin, and be confident that lots of people will buy. But if you're going for big core rulebooks, you need to fill them with suitable material, and that almost guarantees rules bloat baked right in to the core.
Balancing these two competing pressures is an exercise in squaring the circle. I don't envy them the task.
Actually, I read somewhere (I wish I could remember where) that Core Rulebooks, while undoubtedly the bestselling of any line, tend toward being loss leaders because of the development time typically required.
It makes sense if you think about it. I expect the total man hours spent creating an entirely new edition dwarf the time spent creating an individual splat book by an order of magnitude.
I can't cite you a source though, so it's possible I'm wrong.