The default answer is "fear"... but it is often more complicated than that.
Sometimes, it is "time". With all the hours spent at work, simply getting through the task list, and then the remaining hours spent with personal obligations (equally necessary and important), we run out of time to prepare our negotiation. Because asking for what you want, even if you're assured of getting it, is a negotiation.
Sometimes, it's hard to put an "ask" in language that doesn't sound like "gimme!" or "just trust me" - neither of which often work once we pass the age of three.
Sometimes, I don't even know what I want; I just know what I don't want. So it becomes harder to figure out what to change beyond knowing that change needs to happen.
Really, I think what holds back most of the folks I know (including me) is making a priority of framing what I want and booking the time to plan and ask for it. It is easier to wish / complain / hope then put yet another item into a crowded week.
I don't know if that's a gender thing - I see men and women both not making time for their own needs and desires. I wonder if it is because we're taught not to ask?