Because free software community members have friends and family that don't care about software freedom or privacy so if they want to communicate with others, they need to use widely used IM software. You are lucky if all your contacts use jabber, but it's not the case of most of us. If the best world is not possible, it's good to at least have a better one, isn't it?

Telegram clients, protocol, and API are open. Servers are not open. Promised privacy is fine. Obviously you can distrust what they say, but ¿why you trust the jabber server you use? Do you know personally to all people that have access to that server? Have you read the code and monitored the binary generation and use of the jabber server software you use? If you can't trust anybody and/or anything...

Why anybody will take the care to promise privacy and openess, while most people use the less open and less privacy protector of the IM softwares (Whatsapp)? They just want to get the data of the paranoids and free software community members? Are we so important to make that evil movement to get our precious data? Maybe , just maybe, other reason could be that they are saying the truth and doing their best...