It's probably because modern tweet IDs are larger than a 32-bit integer. Presumably some JSON parsers aren't too hot on parsing bigints, so they give you the option of having a string instead.
In Javascript (as well as Lua and PHP) ints are -- under the hood -- doubles. This means they can only represent a 53-bit int: outside that range they start to alias. You can see this in effect in a JS REPL:
