Why? If they make it so obvious of course there will be outrage. What everybody with some brains would actually do is slow down the approval process and really look for all kinds of unimportant small details that the update doesn't fulfill "yet".
So for Apple that is either a good way to get rid of an unwanted app, or a way to do a favor for a competitor, or a way to put pressure on Telegram developers to do something that the devs haven't done yet for some reason (e.g. give access to user data).
Now I'm not saying that Apple does anything like that. It couldn't be proven by simple people like us anyways. I'm just providing possible explanations for what's maybe going on and why it might happen. It is for instance just as possible that Telegram is not updating themselves, because they want to achieve some kind of hidden agenda, e.g. in a subgroup of people they care about users would very much be willing to switch phones just to stay with their secure chat app and they might have a better deal with Google than they have with Apple. Nothing can be said for sure without insider information.