Its pretty simple, I think. They haven't pushed them in the MCU yet.
I mean, think about it this way, if you ask the average person to name a female superhero from Marvel they'd say 'Black Widow', she's currently their most famous superhero to a mass audience.
Now, for perspective: DC is only really massively popular in the US. Outside of the US people know who they are, sure, and know Batman and Superman but growing up in the UK it was always Marvel that was the most famous and everyone wanted to be (anecdotal, I know), that's why if you look at the sales data for superhero merch, Spider-Man is massive abroad even compared to Superman and Batman. I know who Wonder Woman is but I really have no relationship or understanding of her beyond the basics. That's changed now that the movie is out.
Marvel has great Super-heroines, I'm sure, but they've yet to push them in the MCU, the series which is undoubtedly the vehicle for making even unknown superheroes household names.
Harley Quinn's popularity after Suicide Squad still alludes me though, no idea how that happened.