It's a good idea, and I did consider it, but it's been "playground" for 7 years and even third party versions kept the playground name to let people know it's a REPL. Breaking that is a pretty big change which I opted to not do.

Your choice of naming matches mine too, the abstraction which lets anyone build their own TS repl is called the sandbox: https://www.typescriptlang.org/dev/sandbox/