I assume because they couldn't think of a logical distinction between Amphitheaters and Theaters and why the former should have a great work slot and not the latter.
Zoos started to rise in prominence around 1750, which is obviously later than 1454, but it probably came at a good time for balance reasons and they probably didn't feel like changing it. Timeline-wise, economics might make more sense. If you wanted an appropriate tech, Scientific Theory or Biology would make a lot of sense, but I can understand not wanting to wait that long (and Scientific Theory is good enough already).