I've discovered that US carriers don't like you to have that info for some (cough) security (cough) reason - WN being the exception since the tail number is on the outside bulkhead many times. Never mind that it is visible except for the catering truck in the way, etc. However, carriers in other countries don't seem at all to mind giving you that info. I once asked on a BA flight, and got a very nice note back from the flight deck crew with a lot of detailed info on it.