Dual-statting has a couple of issues for writers.
First, the extra work. I'm writing the Cimmeria book right now, and d20 character stats work very differently to RQ stats, so there'd be a heap of work involved in effecting a dual statted book. That's what we're paid for, of course, so it's doable, but I know that if I knew I had to dual stat every major NPC and creature, I'd be groaning inwardly.
Second, the page count will be impacted. It either increases, which pushes-up the price of the finished book, or stays the same, which means that usable content is reduced, and consumers still find that they're paying for a book that has x% of content that doesn't really apply to them.
You could always leave stats for one game out of the book and publish as a PDF that's offered for free, but that, again, raises other production headaches.
So yep, dual statted books are possible, but there would have to be a trade-off and I'm not certain it would be a necessarily acceptable one for everybody.