Why was ark survival evolved removed from the App Store on mobile

No idea. You'd need to contact the App Developer, who may have dropped support for the app.

Edit: A quick Google search and I saw articles suggesting the app hadn't been updated in at least two years, so my thought the app developer was no longer supporting the app may be the right reason.

They are revamping the entire app & and gameplay. Join their discord for updates. Some time in the middle to end ish of December they will re launch it. It will be different, better! That is why it is not available to those who were not already playing on a server before the revamp begun. A few friends of mine still had the app downloaded and have been able to still play just fine. As for others I know they can not due to not already having the app before the revamping / update to the game begun as due to them removing the app from both play store AND App Store.