I'm not sure where the "first priority" bit came from; I was merely suggesting that they would prefer it.
Ignoring any other constraints or considerations, it is obviously more administratively demanding to operate an a/c within the jurisdiction of two separate, and probably far from harmonised, national authorities. Operating on a local registration and AOC etc means a single authority to report to.
Looking outside of that consideration, if an authority allows pilots to operate to less restrictive FTL schemes, then generally speaking that means you need less pilots. You don't need a double degree in rocket science and brain surgery to work out the benefits of that (and many other alleviations)!
