It's really hard to describe how all of this works without going into extremely detailed scenarios and edge cases. Simply put though hiring more people doesn't do anything.

Those extra people still would have to be sitting on call somewhere. Once they receive the call they're sent to where they're needed which still results in this situation. It's simply unreasonable to think they would have spare flight crews in every single possible destination. Unless you want about 1,000% increase in ticket prices of course.

I'm sure the other flight was delayed. Not getting a crew there on this last flight out for the night though would have likely meant cancelling the flight in Louisville or delaying it at least 24 hours. Stranding a plane in a city like Louisville would cause major ripple effects down the line resulting in probably another 10-15 flights being delayed. If they cancelled rather than delay now you have a plane full of passengers that need to be re-routed on multiple other planes that are likely already full.

Re-location of flight crews is simply a necessary option for the airlines at this point of time. Maybe down the road we can remotely pilot airliners and then situations like this won't arise.