EA are probably not willing to finance them for more without wanting them to raise the price. For $10 they are probably pretty limited in what they can do. If EA would put in more money then they would expect something in return because they are not going to change their finances just because people want spiral staircases. It's possible Maxis do not want that either, how much that should be included in a stuff pack is an agreement between the developers and the publisher, Maxis have said how much they can deliver and EA is providing them with money to do it. A larger budget generally means that Maxis have more to deliver.
So when they say "not enough resources" they mean that they would have to charge you more or change their agreement with their publisher. They are not really willing to do that for spiral staircases... Hopefully they have decided instead to include them later this year as a free update.