Meanwhile, Sandwich is the minimum operating system version required to run Telegram’s latest version. I think this move is made to accommodate new features on the Telegram app which older android versions might not support. The same reason WhatsApp is adamant on following suit.
But if you are a Telegram user or fan still running the Android older version, you can still access your Telegram account via the web from your mobile browser or PC web browser. The same applies to whatsapp users.
Read More on their website.