Because past experience matters and Facebook has failed quite a few times on the privacy spectrum
Shiny stickers and broader user market vs privacy.
Almost constant uptime due to being owned by a multi million/billion dollar company vs a group that mainly runs on donations.
Etc etc
I really hope it's the former, instead of the latter.
Whatsapp is a non starter for me because the company that owns and operates it is not trust worthy. Very profitable, and probably making some of the people here great returns on their investments...but it's not trust worthy, nor ethical.