How do you think they come up with the budget number? It generally has to be set with an idea of how much money they are going to take in. If they are doing their job right (which in some cases they obviously have, since they fill the park), each year they are expected to increase the amount of tickets sold. Over the years, do you think their budget has increased or decreased as the number of attendees has grown? Whoever is in charge of setting that budget decided (possibly) that an extra tent wasn't going to sell enough tickets to be worth it, especially when there is room elsewhere that costs basically nothing. However, using Shrek is nothing more than putting a band-aid on this space issue. What are they going to do the next few years to keep those profits up? Maybe they just weren't ready yet to make that investment, or maybe they think of it as a card under their sleeve to cash in at a later date.
To the people above who are in charge, profits are all that matter. For those who are handed the budget, they are forced to make it work.