Why do primitive animals still exist?

Because more complicated doesn't always mean better at survival. In some places, simple works better.

They exist because they are very well suited to the habitate they occupy or they exist because they are highly adaptive to changing habitates. "Advanced" is a subjective description. How do you define advanced? For that matter, how do you define primitive?

there is still a niche where they work

coelacanths still exist cause in the deep waters of the indian ocean (where they live) conditions are the same as they were 200 million years ago... cold, dark, and without alot of food