It's unfortunate that there were not enough shirts for everyone. In the F.A.Q. section for Nationals it does specifically state that the shirts would be available while supplies last.
It is always hard to predict the total turnout of any tournament, much less the numbers in each age division or the sizes that will be needed for those divisions. There were actually more shirts this year to give out than there were last year; however, there were also approximately 200 more players this year than last year - a very large increase of players that was hard to anticipate.
TPCI has always been very good about trying to provide supplies to players free of cost - no small feat when you consider the shear number of players involved, and will no doubt use the numbers and feedback this year to better accommodate players next year where they can. My best advice is to arrive and be present at the very beginning of the registration process. I'm sorry that, along with many players, yours did not get a promotional shirt. I know that doesn't help for this year's National Championship, but should you happen to participate in a National or World Championship in the future you'll stand a much better chance of receiving items like this by doing so.