Because they didn't need to. They survived just fine without changing. Evolution is not about going from simple to complex or from "worse" to "better". It is about surviving in your enviroment long enough to reproduce
You're confusing survival with evolution. Evolution means change. 500 million years ago our ancestors were bony fish swimming in the same environment as the jellyfish. Supposedly the jellyfish and the vertebrate fish had access to the same evolutionary mechanisms. Yet some fish became amphibians, reptiles, birds and mammals while the jellyfish remained jellyfish.
There is a mechanistic answer to this ignored question which has been pubished in a peer-reviewed journal and you'll find it here LINK
There are some serious problems with evolutionary theory (I was a minor in anthropology and studied et at length).
Imagine my surprise when I discovered that some evolutionary "scholars" (Stephen J. Gould comes to mind) were perpetrating LIES - such as the spotted moth story.
Curiosity piqued, I started a study and found all kinds of anomalies in et. I'd suggest that others, similarly interested, conduct their own research into how often - and how pathologically - the Left lies.
Better watch out, though. I became a diehard conservative!