From reading many stories about Apple product development, it seems like there's a disconnect between the product teams inside Apple, and even with the iOS team.
The small iPad Pro was probably developed to be just an iPad Air 3, not an iPad Pro. I mean Apple didn't just churn this thing overnight, it was probably already in development a year or two ago, and during that time, even the iPad Air 2 can run iOS9's multi tasking, so this iPad Air 3 is simply a reiteration, with just the SoC and camera being upgraded. But then came the iPad Pro, and since it was going to target different market (and more room inside), they maxed it out, and it was probably too late to redesign the board for the iPad Air 3/Small iPad Pro for additional RAM.
I would say the next one will have 4GB RAM. This is a transition product. The current iPad Air 2 will be the "iPad 2," that it will linger around for few more years as the lower end 9.7" iPad, while all the goodies will be for the "Pro" lineup.