You should try convincing them adding more expansions next year. I know it is a cost issue.
Cost should be a minor issue for any official tournament, unless there are ridiculously many players. The Finnish tournaments, organized by the local publisher/distributor/store, have always used the latest expansion and then 1-2 of the older ones. They provide the games by using existing demo copies, copies brought in by participants, and by opening new copies if needed. Some of those they retain for demoing and future tournaments, but if too many copies are opened for the tournament then they simply sell the excess ones for a small discount (either at the tournament or afterwards at their store). Whatever they lose in opening those copies is negligible and probably dwarfed already by the prizes or by the personnel costs for hosting the tournament. The tournaments are always 2-player Swiss tournaments, and at least the latest edition I went to had each pair playing best-of-three using the same kingdom (shared for all pairs) to further reduce random variation.
I can, however, hazard a guess on the real reason for hosting a base-only tournament. For companies that are not "within the community", hosting a tournament is primarily a promotion event. They want to attract a lot of people, including very casual players who do not yet own the expansions, not just the experts. They want to be able to sell more of the expansions, so requiring participants to know them in advance would be counter-productive. It also sounds more convincing when they can say they hosted a 200-player tournament, instead of a 12-player tournament where a few semi-professionals played against each other, even though the quality of play would naturally be much higher in the latter and the winner would be more worthy of the title.
In other words, Dominion is simply not at the level of an official competitive sport. You can either host a competitive tournament for good-will towards the expert community, or a bigger one for largely marketing. Mixing these two is difficult.