WhatsApp also lacks multi-device sync. The feature that made people adopt WhatsApp was saving money in contrast to using expensive SMS (which is the reason why it doesn’t have much adoption in countries with reasonable prices).

Multi-device sync and privacy-awareness both are maybe reasons for a few individuals to switch but they both have problem with the number one feature of messengers: actually reaching other people.

Why something gets adopted is way more dependent on soft factors than on technical factors, as long as the basic requirements are satisfied.