I think part of convenience is name brand recognition. I don't know how you took a heartfelt compliment and made it hostile, but the reality is I grew up knowing what Google was and using it as a verb. Gmail was an obvious and convenient tool to pickup.
I just found out about Protonmail, or at least heard of it for the first time that it broke the barrier of not-caring into carrying. I imagine user numbers reflect that pretty readily.
That's all I'm saying. I'm not saying Protonmail is worse in anyway, please don't assume I am. It's okay to like a product and admit it's flaws, in this case the only flaw I'm suggesting it has is being less known than Gmail and even then only for me and my small corner of the world.