When the idea of adding a mount system was first presented, the dev team itself was divided. There was a fair amount of discussion about the historical accuracy of infantry in Greece, and seemingly endless discussion ended in an official forum vote. The fans turned out to be "overwhelmingly" in favor of mounts, and production got underway.
Again, it wasn't a simple and quick process: "There's a lot more to it than throwing a character on top of a pig." Many, many small effects work together to create a good mount, and each of those effects must be perfected individually.
This journal entry is a great look at what goes on behind the scenes during the creation process, so it's well worth a visit to the site to check it out.