Well there are two questions in your post, why not more scenarios and why not 1900/2000 AD.
As for the first part, it's just a lot of work to make another scenario, especially if it is later in history. Creating the 1700 AD scenario probably took ten times the work that 600 AD did, because there are more civs, cities, units to place, techs to assign etc. Also every scenario added increases maintenance burden: if I decide to change something, I need to do it for all scenarios (in most cases), so every additional scenario makes that more time consuming.
Whenever I'm changing something about the game I'm trying to make it easier to create and edit scenarios but that's still not at a point where it becomes easy.
I'd like to have 1-2 additional scenarios in the mod, but I don't think those will be later than 1700. In my opinion, this game is about the rise and fall of civilisation, and the alternate history that arises during gameplay, either while actually playing or during autoplay. 1700 was added in part because we had Brazil (or maybe Canada already) which took ages to load, but they're fine to reach through 1700 AD autoplay. Also, I don't think the modern game is very balanced and fun, so having a scenario for that timeframe would expose those problems more.
Actually if I add another scenario, it'll probably be around 1500 AD, being later than 1700 AD but not having European expansion across the world hard coded in.