This is not something that the moderators can do on a per-site basis. So, I am not really sure if there is much we can say about this matter.
Scrolling through meta.SE the closest related question I could find was this: How do moderators permanently add Site Chat Room to main page sidebar?How do moderators permanently add Site Chat Room to main page sidebar? which I believe Loong already linked you to in chat. Of course, you are most welcome to post your suggestion on meta.SE.
Personally in my experience I cannot find any logical pattern as to when chat shows up in the sidebar. As such I always access chat via the Stack Exchange logo in the top-left: