(I don't know why, but it was the most popular distro in the pine64 poll)
If you look at the results, you will notice that Manjaro is not the most popular distro for people who use the phone as a daily driver. This indicates that people who keep Manjaro are less likely to actually use the Pine phone as a phone.
So it seems that the reason it is the most popular is that is the default. If you look at what interface people like, the default (Plasma) only scores 21% in that poll and it falls to 18% amongst those who use the phone as a daily driver.