Petrie15,
Cluttering a story with too much stuff isn't really a good idea. Sometimes, it gets to a point where things just have to be cut. The story needs to be able to flow in an understandable way, or else you risk losing the audience.
What purpose would showing reptiles have had? Showing diversity? They already accomplished this with the wide variety of mammals. Adding reptiles wouldn't have really added much to the story. Perhaps a sequel can work them in with a different plot, but the plot of Zootopia and its setting did not really require reptiles.
While inclusion is nice, things added in a story have to have a purpose beyond just trying to appeal to a minority.
And as pointed out before, excluding something isn't really an attack or a statement against those groups. Streamlining a story is important. While you don't want a story to be lacking in substanence, you also don't want it to be overflowing with too much stuff either, as that can do harm in its own right. And the reasons for exclusion can vary, but it isn't always a jab or because they don't like it. It could be because the material is not necessary for the story they want to tell.
If not having reptiles causes Zootopia to be a deal breaker for you, Petrie15, then okay. As pointed out, there are other options. For instance, Sing, which I posted a trailer to earlier on, which has reptiles in it, not just mammals.