This happens because whatsapp stores all the information in your phone itself. On the server side, at least before FB bought it, they didn't store your info. this is the reason their team was so small despite the billion users, because they just need to store data on a need basis, since they were a paid messenger.
The moment you switch phones, all the data is lost, because they do not have it in the cloud! This is the reason your phone needs to be connected to Internet when using whatsapp web or whatsapp desktop, because they literally stream your data i.e. chats from phone to desktop!
Telegram is a cloud messenger, data is stored in the cloud and loaded everytime in your devices, ironically this makes whatsapp better than telegram :D at least before facebook bought it.
The moment you switch phones, all the data is lost, because they do not have it in the cloud! This is the reason your phone needs to be connected to Internet when using whatsapp web or whatsapp desktop, because they literally stream your data i.e. chats from phone to desktop!
Telegram is a cloud messenger, data is stored in the cloud and loaded everytime in your devices, ironically this makes whatsapp better than telegram :D at least before facebook bought it.