Does a zoo need a constant influx of new exhibits? Is it sustainable to constantly build large-scale, complex exhibits, and require heaps of donated or public funding? Wouldn't that start to alienate your benefactors, especially when your enclosures are quite good? Isn't there more to a zoo than just architecture? What about collection? Conservation programs? Temporary exhibitions? Aren't these zoos supported by communities and thus should cater to them?
To answer your question regarding capital projects:
Since 2008, there has been quite the economic recession that Florida was particularly hit by. Meanwhile the zoo's director focused his attention away from the zoo and towards his own extracurricular business activities. The zoo lost public and donor support. They needed a restructuring in administration and finances. And finally a need existed to update their behind-the-scenes support facilities before moving forward with any further expansion.
In the same time period they've bred 2 rhino species, African elephant, Okapi, Shoebill, and Clouded Leopard. Increased success with Manatee rehabilitation. Added more species that are threatened with extinction to the zoo's collection. And still seen over 1 million annual visitors. This is considered stagnant?
Sorry but there is a fundamental problem if zoos are only growing/reaching success when they build new exhibits.