You might have noticed that lions (and other big cats) twitch their ears to get rid of the flies or when they're nervous. But why do you think TLK lions don't do it? Did the animators forget about it? After all, they're animators and not zoolgists. Or did they deliberately omit it? What are your thoughts?
Probably because it wasn't important enough to be included, that and they aren't zooligists.
The Lion King franchise often plays fast and loose with real lion behavior. Most of the time I believe it is because certain aspects would get in the way of the stories they want to tell.
Why do we fall? So that we can learn to pick ourselves back up again.
By and large the TLK universe opts for a pretty sanitized version of Savannah life there's no ear flicks because there's no swarms of flies around lions as they eat, or for that matter many meaningful depictions of a kill for the lions to eat.
Personally, I assume they just did not have the time to focus on such a small detail. While I agree it would have been a nice touch, animating (especially when hand drawn) is a lot of hard work and it would be hard even for a team of professionals to add in such small details on a deadline.
I agree with what has already been said, otherwise I wanted to add that it`s not a twitch of the ear exactly, but adult Simba does turn his ear before turning his head when he hears Rafiki singing - I`ve always enjoyed that little detail, a realistic animal behaviour.