They probably don't care enough about a sidebar because they don't know about it or it doesn't come to mind. How many users are going to go to a subreddit and think "I really need to go check out the sidebar!"? Most of the time it's there to try and improve the posting quality. If using a mobile app is a user's primary method of viewing reddit, how would the sidebar even enter their minds?

The sidebar being reachable from the context menu is nowhere near sufficient, and since this is now an official reddit app, they should care enough to promote a feature that improves the quality of reddit.