We've avoided dual stat books for a few reasons; here's three that come to mind.
1) We've pretty much thrown all-in on supporting 2nd edition. I'm proud of the work we did in 1st edition, but I personally feel that 2nd edition is more enjoyable to play and to write for. Its improvements and changes are the result of us trying to make a game we prefer, after all, as much as us trying to make a game for the rest of the world. We WANT to produce 2nd edition content.
2) We can't change the laws of time and space, and so if we did dual stats for our books, we'd have to either make fewer books or make them shorter. At the same time, the effort to design and edit and layout dual stat books is not twice as hard as single-stat books. And even if we did decide to simply expand the books by however many pages it would take to do include 1st edition stats, there'd be customers who would be disappointed that the pages devoted to the system they don't use are "wasted" and that we could have given them more content in those pages, so it can foster complicated customer relations.
3) It wouldn't be twice the money, but it'd be more than twice the work.
All that said, for lore-heavy books like this one, it IS in large part edition neutral, and GMs who want to expand upon the Mwangi in their games or are looking for some more lore to fill their own campaigns will find a LOT in here to be inspired by. Regardless of what game you play.