From Apple,
About Messages via satellite on your iPhone
With iPhone 14 or later (all models), you can send iMessages or SMS messages via satellite when you’re off the grid with no cellular and Wi-Fi coverage.
And by "off the grid with no cellular and Wi-Fi coverage" Apple means you must have zero cell signal bars. If you have even 1 cell signal bar.....NO satellite messaging for you. That's how it is currently configured by Apple.
So there you are at several thousand feet, somewhere over the US. You might have just enough intermittent cell signal for the phone to not make a satellite messaging connection.....but not a consistent enough cell signal to send a cell based text message either.