It will probably get worse when the players that only play Planet Zoo to collect CC move on to other games. Most of the time these are no long time players. But at the moment I'm pretty sure the only reason for the lack of animals is, that over the holidays many players didn't find time to play and put animals on the market. I don't know for sure though, I don't play franchise anymore.
Honestly, the day the CC tycoons leave the game, is the day I start playing Franchise mode properly.