Why do (some) play leona passively?

Because I don't know you (ADC) and I can't trust you, or communicate with you effectively. Ergo, I'll wait for the enemy ADC/Support to make a positioning mistake and act on it. So be ready. Until then -- focus on last-hitting.

I think it mostly comes either from a lack of communication between the adc and support or because the leona player does not trust in their ability to initiate on their enemy. I've been yelled at before while playing leona for not being aggressive, even though I get half healthed by their adc before my own reacts.

It mostly depends. After one or two E->Q dives you can get a feel for the lane and how you should play it. If your carry doesn't follow up and you lose half your health, or if your enemy is also playing safe where you'd have to over extend in order to E them, then the best choice is to remain passive and protective of your ADC. If they do follow up and you get your enemy at half health, or even first blood, then it'll be a good lane.

In short: Passive ADC -> Passive Leona, Aggressive ADC -> Aggressive Leona

Because some people (me,) are not good with Leona, yet the team gets red in the face if I don't go for a tanky support.

She's actually a pretty good babysit support on purple side.

Leona is probably the easiest support, so she is used by a lot of non support players who are forced to support. That is my best guess anyways.