IMHO because it means that Signal stays focused on being a secure messaging platform and not an 'application platform'.
Telegram is great because it has bots, etc. but Signal is great because they have one core focus and they don't complicate their infrastructure or codebase by adding things that aren't what people want from Signal.
There's nothing wrong with having two messaging applications; let Telegram be the bot and automation platform of choice and let Signal focus on being a secure and reliable messaging solution.