Yes, there has been.
When my plant clubs got kicked out of their botanical garden meeting places it was because instead of getting $200 from us they could get several thousand for weddings. I can see why zoos might go after this kind of income. Plus there is a backlog at a lot of other possible venues, at least where I am.
I hate this by the way. Inevitably half the place gets taken over by the smug bride who has no idea how to wear her hideously expensive, and typically only worn once dress), her idiot wedding party, rude guests, and pushy caterers and wedding planners. They will even try to push you out of places that you are allowed to be in (I have a really funny story about a wedding planner trying to keep me out of the only public bathroom at an art museum).