For many it was the prospect of metadata being sent to WA's big brother Facebook.
Personally I don't want anything to do with the FB ecosystem so I deleted my WA account. I didn't have anything against WA per se, I really like the app , just the connection to FB is unacceptable.
But if telegram sells to FB or someone worse, you will be in a worse position as now they will not only have your metadata, but also the full text of your messages.
It doesn't make since to move away from one company for privacy reasons onto another company with even worse privacy.
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.