Usability; and I hope I indicated clearly that it was a personal detestation over an objective claim :)
for what little it's worth: Facebook Messenger, Hangouts, Google Chat, let alone messengers of yore like ICQ, MSN, AIM, etc all allow me to create a userID & password on computer and use them from any device of my choosing, as well as change/transfer/obsolete devices seamlessly.
Whatsapp is bound to my phone number and device, and its experience with multi-device is.... actively hostile. My preference is to not type messages on a 1.5" keyboard, but rather on any number of ergonomic keyboards and devices I use around the house and work. Whatsapp is simply prohibitive for my usage, and for positively no actual (as opposed to theoretical/principled) benefit whatsoever. I've pulled what little hair I have trying to use it, but the 27th time it locked my account for using too many devices (Oh noes! I have a phone, tablet AND a laptop? Crazy me:), I gave up.
(privacy wise, also, my phone number is one of the more private and static pieces of identifying information I own; I guard it carefully and I have no idea why I need to expose it to everybody I want to communicate with, as opposed to anonymous and disposable userID or even email).
I did not mean to turn this into anti-whatsapp rant though; Many other E2EE-focus services are similar. Whatsapp just happens to be wildly popular and, for my use-case, completely and actively impractical :-/