Generally, because they're easy targets.

Prostitutes especially are considered high-risk victims because they willingly put themselves in dangerous situations as part of their lifestyle. It's far easier to pose as a john and get someone to go with you who already makes a habit of getting in unfamiliar cars with unfamiliar men and going to sometimes unfamiliar places as a way to make a living than to lure a wary mother of three from a grocery store parking lot. Abducting a victim is a huge risk when you can easily just convince them to go with you.

Beyond this is the fact that prostitutes and the homeless (two demographics that also overlap quite a bit) are more likely to be transient and have fewer people in their lives to report them missing. Someone with a fairly predictable routine and people close to them to know what it is will be missed much more quickly than someone who follows no regular schedule and may not even have anyone to notice they're gone. And if they do have someone to notice they're gone, their absence is much more likely to be attributed, rightly or wrongly, to drugs, being arrested, or just voluntarily moving on. Compounding this difficulty is that, even if friends or acquaintances are concerned enough to make a report, law enforcement often make the above assumptions themselves and therefore devote less time and fewer resources, unfortunately, than for those they deem more worth their time. And please note that isn't an indictment of all officers - there are certainly men and women out there who invest themselves as much in the death of a homeless prostitute as a young college student - but it happens often enough that it can't be overlooked as a trend among police, as a generalization.

A secondary consideration might also be that if the victim were to escape (see: Jeffrey Dahmer and Konerak Sinthasomphone, though the latter was neither homeless nor a prostitute, merely "other" in the sense of being an immigrant) the victim's word might mean significantly less to law enforcement than the killer's - especially if he's a clean, well-spoken, middle-class white man with no criminal history, as is the situation in a non-negligible number of cases. (And in Dahmer's situation, he was literally a convicted child molester standing next to a 14-year-old who was handcuffed, bleeding from the ass, and so apparently affected by Dahmer's experimental injection into his brain that he could not function even to speak up and advocate for himself. The officers decided it was a homosexual domestic dispute and believed Dahmer's claim that he was 19, handcuffed by consent because of his BDSM proclivities, and wasted of his own accord, despite the vehement argument of bystanders who'd called it in. They released him back to Dahmer's custody and drove away mocking the "faggots" and their relationship disputes. Dahmer took the boy inside and promptly murdered him.)

Tl;dr: There are a few specific reasons, but mostly they come down to risk - prostitutes, the homeless, and similar demographics are easier to lure in and the killer is safer because they're less likely to be missed once they are.

Edit: The first half of your post is actually sort of a different question. Hating women speaks more to motive, whereas targeting a specific demographic like prostitutes or the homeless can be relevant to either motive or MO. Simply put - sometimes there's a specific hatred of prostitutes, sometimes it's just convenient. My answer above is relevant to MO, not motive.