Presumably the fact that Facebook owns WhatsApp and continues to erode all the "protections" users of WhatsApp initially had from the invasive tentacles of Facebook into their data on WhatsApp.
I do use Signal when possible, but I find WhatsApp to be secure enough for my day to day messaging with friends and family.
I can't say the same about Telegram since it does not have E2E encryption on by default on all messages.
It doesn't make since to move away from one company for privacy reasons onto another company with even worse privacy.