The reason is that the Telegram has shown a "willingness" to help in the fight against terrorism.

Russia lifted the ban on the Telegram, which it had failed to enforce in two years of existence. According to the local Roskomnadzor, however, the reason is that the Telegram has shown a "willingness" to help in the fight against terrorism. Which originally required Russia to Telegram share encryption keys to allow access to users' content. The telegram refused and began a tug-of-war, including such strange ideas as blocking 18.6 million IP addresses, affecting the accessibility of services from Google and Amazon and problems for Russian companies. Blocking VPNs or anonymizers didn't help either. See Russia lifts its ban on the Telegram messenger app.