One reason is that the US Senate is such a sewer. I'd think Cruz would be necessary there to help Jeff Sessions clean up that mess.
You are right. For a variety of reasons Cruz may be better placed elsewhere (Supreme Court?). He’s be a good VP, but Trump may have someone else in mind. Will be interesting to see how this all plays out, but for right now, getting Trump over the finish line is all important. The long knives are out to get him this very minute.