Different people process information in different ways, and a lot of people are going to find multiple contexts useful. Everything else is largely noise.
To be all things to all <gender neutral pronouns/> or concentrate on a subset of users that are happy enough with "classic" GTD or don't even use contexts.
The product is already pretty complex (but not unnecessarily so IMO) and adding more features would only make it more intimidating to new users and harder to support/sync on iOS. Multi-contexts (or tagging) significantly affects the UI, workflow and scripting, it would break a lot of stuff.
If they did something radical like abandoning contexts and going with tagging that would just upset all the existing core users who like things as they are.
I've road tested "Things" (which has hierarchical tags, v similar to multiple contexts) and while I liked that feature, OF did everything else (that I needed) better.
With Things 3.0 about to drop it's possible that there will be a home for both camps. With two task managers of similar quality levels but competing on approach/metaphor (tags v contexts) time will tell where the new customers go.