Why does Typescript use the keyword "export" to make classes and interfaces public?

The primary reason is that export matches the plans for ECMAScript. You could argue that "they should have used "export" instead of "public", but asides from "export/private/protected" being a poorly matched set of access modifiers, I believe there is a subtle difference between the two that explains this.

In TypeScript, marking a class member as public or private has no effect on the generated JavaScript. It is simply a design / compile time tool that you can use to stop your TypeScript code accessing things it shouldn't.

With the export keyword, the JavaScript adds a line to add the exported item to the module. In your example: here.SomeClass = SomeClass;.

So conceptually, visibility as controlled by public and private is just for tooling, whereas the export keyword changes the output.

A few things to add to Steve Fenton's answer:

• export already means two different things (depending on whether it's at top-level or not); making it mean a third is probably worse than adding public/private
• It's definitely not to make the implementation easier; the added complexity of public vs export is trivial. We've changed keywords around a bunch already; it's not difficult.
• The default visibility of class members must be public to align with the ES6 class proposal, therefore we need some keyword to indicate "not public". There isn't a suitable antonym to export (unexport??), so private is the logical choice. Once you have private, it would be somewhat insane to not choose public as its counterpart
• Use of export to modify visibility in internal modules is the best-guess alignment with ES6 modules