Most likely because it implicates NATO.
Apparently, from what I've read, it's also because Poland seems to expect us to "reimburse" them for the MiGs with bright, shiny new jets. All I see is Poland wanting us to
give them jets.
Give? No, Poland ask that they be replaced with old used F-16s (which are actually older than the MiG-29s). Blinken had already indicated that the idea of replacements was acceptable, and that the details were being worked out.
It was the US State Department that came up with the plan of Poland turning the jets over to the US first. When Poland agreed, the Defense Department stepped forward to block it. So now if Poland turns the jets over directly to Ukraine, they won't be getting any used F-16s. Which means they'll be down a couple of dozen planes with a hostile Russia inching closer to their border.