I think a new bus line to a zoo is something easy to lobby for. There is a win-win situation - the bus line gets the tickets, the zoo gets extra visitors. There are relatively low 'sunk' costs.
Bus routes are something more likely to close than open up. Whipsnade has lost their only decent direct bus route, leaving them with an indirect route that requires a change of bus and a very poorly timed last bus from the zoo. There are probably others with just as bad a service.
Some zoos put on a shuttle bus to and from the nearest public transport hub. These only one I've used (about 4 years ago now, so things might have changed) was Paradise Wildlife Park, which would shuttle visitors in a minibus for a small return fee from Broxborne train station.