I think Signal and Matrix are more trustworthy and auditable.

However I also think more people are on WhatsApp than those two (and possibly more) combined and people want to just use it probably for that reason.

I saw some of my contacts sign up to Signal firstly after the privacy fiasco then again in smaller quantity after the major outage. I deleted WhatsApp a while ago but I decided to re-install it again recently or basically risk losing contact with some old friends. In fairness I did try and convert people to Signal; I managed to convince a couple - so not all was lost.