[torresjrjr]

A single ':' is used for other things, like casting types.

    let x: int = 0;

It's so that the compiler can distinguish between those two, and likely some other areas too.

[xigoi]

A different character could be used, such as # (like in VimScript) or \ (like in… PHP?)

[torresjrjr]

True, but :: is probably more readable than # or \ Design a language, and you'll understand that there's always trade-offs.

[xigoi]

I personally find :: very unreadable, for the very reason I mentioned.