The reason I say "4" even though he had 5 wives is because Hortencia married him before he changed. But the other 4? Don became verbally abusive as soon as he was possessed so I assume so did Carno. Hortencia may not have had any way of knowing what went on, but the other 4 wives never knew him before he was possessed. They may not have known he was a murderer before they married him, but you'd think those 4 women would have always known Carno as a mean, abusive man. I wouldn't want to marry someone who constantly abused me, even if it weren't because of a demon.

MINOR SPOILER ALERT.

Didn't Lou Ann from the Antique Shop tells Adrienne, his wives worked for him in his show?

Since Carno was into black art, maybe he did something to them: tricked them. Also, his wives where very young so maybe he could more easily manipulate them. Put on the charm long enough for them to fall under his spell. Or maybe the wives thought they could change him, or he would change for them or thought naively: "He loves me, he wont do that to me". Also, different times when it came to the position of women and the pressure/expectation/norm to marry. His fifth wife, Marie, does come to the realization that Zoltan has been murdering his wives.

Of course I'am speculation. It's been more than 20 years since I finished the game so I can't remember anymore if the games offers an answer/explanation to this question. Now I'am almost entering chapter 4 so I guess I will have to play and see. ;)

EDIT:
Oh wow, didn't realize this is a very old topic.

That's fine. I'm always happy when people respond to my topics, no matter how old they are. What's kind of depressing is when no one ever responds to them. I'll take a late response over no response any day. I've responded to some old topics myself as a matter of fact.

I have since considered another thing, although all the things you said are great points: Carno was the richest and most famous person in these women's profession (Lou didn't say they were all in his magic show. She said all of them worked in theater and some of them were in his magic show). Maybe money and fame can buy you a woman no matter what kind of person you are, especially back then. Nowadays women sometimes make the same or sometimes more money than their husbands, but back then women were always dependent on men to bring home the money. So maybe the women were willing to put up with the abuse to be with a man who can provide for them a great house and never have to worry about money issues. Plus being married to Carno was probably something you could brag about.

lots of women marry horrible men regardless of money or status, like they're blind to it or the men hide their true selves, etc.

add in money and status and a lot of women are willing to look the other way at a dude being an asshole

I've always figured that the reason they all fell for him and married him, was because he was the charming bastard.
A lot of abusers are incredibly charming in the beginning, and considering that the demon lived within him for so long he would learn to live with it in some way, and go more charming than what we see of Don.

As to why, well, there's always the "division of labor" concept that apparently appeals to some women, somewhere...;) With four wives, one wife can handle the kitchen, another the laundry, still another can do the grocery shopping, leaving one wife left to clean house, etc. They'd rotate duties among them, certainly, as well as *cough* conjugal duties every fourth or fifth night--depending on the number of currently available wives, of course.

I keep trying to sell this to my wife, but she's not having any of it...;) Come to think of it, would I *really want* 4-5 wives? Actually--no way, Jose'...;) One is quite enough...;) Of course, this is a joke response to the original OP's query, and I am j/k...:D