Moments gives you a way to stitch multiple tweets (including ones with images and videos) together to tell a story, impart a message or provide event recaps. They're curated stories that live within their own tab. Twitter didn't explain why it's spending time and energy pulling the feature out of the apps if it's not seeing a lot of use in the first place. It's possible that the company came to the conclusion that maintaining the feature for mobile would take too much effort and is more trouble than its worth.

On October 23, we're removing the ability to create Moments on the Twitter for iOS or Android apps. When features aren't used as often, we'll remove them, so we can focus on building other products you'll love. — Twitter Support (@TwitterSupport) October 9, 2018