Can't they notice??

Feminism or something

The easy answer would be: “vaginas.” Being more precise though it’s change. People tend to fear or have aversion to change. Many think that recognizing and including others means less or none for them. Some percentage of people just wont like any movie. I’m sure many who disliked the new Ghost Busters would have disliked it just the same had it been a male or more mixed main cast. But by and large as it pertains to this discussion- a great deal of criticism aimed at these movies is directed at the fact or related points to the choice to feature balanced or strong female roles and leads centrally. Some is just general counter culture lash back. It doesn’t matter how worthy a cause is- many, especially those with adolescent level minds- will natural push back against it if they see it as a “mainstream agenda.” A similar phenomenon occurs when people like something or a genre or what not that suddenly finds mainstream popularity, or when something becomes the talk of the town. Example:

Harry Potter. Many have never read a Harry Potter book, perhaps not seen a single movie, and even if they have may not have honestly given it a chance but decided ahead of time they would mock and dislike it. For many, it is “cool” to dislike and mock Harry Potter simply because it’s so popular. They are standing counterculture against something that is accepted by most- a statement of how avante-gaurd their tastes are, or that they are an individual and so not follow “fads” or popular normalcy. The self identity of “outsider” or “disruptor” is important to many- especially under achievers who have no other accomplishments or features of note to give them a sense of worth or social footing. It also serves as a defense mechanism- to reject society before it rejects you, or for those who feel somehow rejected or scorned by society to “get back” at it by rejecting what it loves.

So many feel attacked, they either feel threatened that by including and recognizing others they will lose stake, or they tend towards more primitive and instinctual views on things and can’t seperate what is instinct from what is considered acceptable behavior in a society. Self improvement and change, overcoming basal urges and fears, adapting and learning can be difficult, and more so for some than others. Like a child who makes jokes in a certain class or makes it a point to be a clown or “bad student” it is less about that vein an aspect of their personality and more about the fact that they find the subject or course work difficult either because they have trouble understanding or summoning discipline to complete it as others do. As such, it saves pride and self image to instead say that “math is stupid” as opposed to saying “most people seem to get this and not have the trouble I do, but I do not seem as able at it and am having difficulties.” It doesn’t help that so many can...

... be militant or hostile in their beliefs or requests for change. This not only can further make people feel the need to become defensive, but can help justify their position that “this” or “these people” are bad. So in a bit shell that’s the more in depth reasons why so many have issues with many of these projects. I won’t get into the individual justifications many try to use to argue otherwise such as that focusing on females is the opposite of equality or any such nonsense. If we want to be brief though the answer to your question is: vaginas. Vaginas are why.

The last Jedi got flak because it was trash. Awful. Horrible. All-around bad movie. There are plenty of video essays on why. And it's not because there's a female lead.
I've seen nothing so far about captain marvel getting flak, probably because it looks good.
People hate ghostbusters, again, not because of the "female lead", but because the trailers were so awful (and they didnt represent the movie; those who decided to watch it did like it) that many people decided not to watch the movie based off the trailers alone.
People believe that replacing an already-popular cast in an already-successful franchise is a cheap, unimpactful way to pander to femimists and SJWs. And it really is - if you want to put some more diversity into your Hollywoods, make a new successful franchise or use one that already has women or people of color, if that's what you're really going for.