My educated guess is that the localization data for every language is pretty massive, so per-region downloads can include the text for their region without dragging along a few extra megabytes of text/images for other regions. If you swap regions after installing it can probably download on-demand.

Historically reducing your download size by a couple megabytes has had measurable positive impact on conversion rates (from webpage visit -> install) so I expect this tradeoff made a lot of sense in the past even if it's less important now.

EDIT: One other thing worth note is that there's zh-CN and zh-TW, and I can imagine the Chinese government being very irritated about the idea of their release having separate Taiwan loc text/images baked in since according to them Taiwan doesn't exist. Major software like Windows has already had CN-specific releases for this (among other) reason(s).