The reason primarily is that they have rolled their own crypto, which based on a lot of reading and comments from other experts suggests is the wrong thing to do.
To this end I havent seen a good explanation from Telegram as to why they have taken this route given there are so many proven & battle tested methods and algorithms. "We havent been hacked yet" doesnt seem good enough for me.
1:1 messages aren't e2e by default and group messages dont support e2e.
For these reasons about encryption I dont like it but their UI/UX is the absolute best.
To this end I havent seen a good explanation from Telegram as to why they have taken this route given there are so many proven & battle tested methods and algorithms. "We havent been hacked yet" doesnt seem good enough for me.
1:1 messages aren't e2e by default and group messages dont support e2e.
For these reasons about encryption I dont like it but their UI/UX is the absolute best.