I have a theory on why: it's because the scope of a general-purpose-ish language inherently increases; there are always users that are mostly covered by the featureset, who just need one more feature and they'll be fully covered, so they'll ask for that feature. But by satisfying them, you'll bring more non-users from their nowhere-near-covered position up to the "99%-covered" boundary, and then they'll want their own last-1% feature.

The same people who want simplicity in their language/tooling also want to not learn a second almost-identical system just to achieve that last 1%, so refusing to expand your scope here isn't necessarily "optimizing for simplicity".