Please specify your question, are you asking wether Telegram does not use end-to-end encryption or does not use multi-party encryption or do you want to know if its difficult to implement?
Encryption is hard and especially group chat encryption is nothing to underestimate.
Signal implementes a flavor of OTR, called Multi-party Off-the-Record Messaging (end-to-end encrypted group chat).
Telegram "Secret" Chats are end-to-end encrypted using an own protocol called MTProto, "Normal" Chats and Group-Chats on the other side are not end-to-end encrypted, allowing users to use multiple devices for the same chat and download chatlogs on multiple devices too.
Seemingly, the developers of Telegram value the usability higher than messaging security.
Take a look at the Whisper Systems Blog Post and the MPOTR Whitepaper:
Also related: Are Telegram secret chats secure assuming MTProto isn't?