Why can't Arizona use it's National Guard for border security?

Answer 1

For one, there'll be political fallout over militarisation of what's intended to be an unmilitarised border. Second, there's the cost - without those units being federalised, the cost of keeping those units active would fall entirely on the State of Arizona. Third, those members of the National Guard are citizen-soldiers - they're not full-time soldiers, and they have everyday jobs which they count on to make a living. Keeping them activated is going to impact their careers, their employers, etc.