It is not just one reason, it is many reasons and many times mothers are struggling with several challenges.

~There are many medical issues beyond what I can go into. For example: some women have "inverted" nipples, and the baby can't feed.

~Women need to watch what they eat. Some foods make a baby cranky. Many things go into breast milk like caffeine or alcohol. One of the hardest things about pregnancy is giving your body over 100% to another human. When the baby comes, there is a certain liberty and freedom that comes with not having to worry about everything you eat, breathe, or do. However, breast feeding still causes many restrictions.

~ Another restriction is separation from the baby. In this day and age, our culture is not structured to facilitate a breast feeding mother. Mothers generally can't take a baby into their daily job. They can only leave a baby in between feedings. Pump machines sometimes don't work; take exorbitant to produce a little milk; is generally a difficult process. Breast milk can be difficult to store and can expire.

~Your body makes milk based on how often baby is feeding or you are pumping. That means that your breasts need attention every 4 hours or so. If milk is not released, breast become huge, painful, and full of milk (engorged). Be careful when you release this milk, you can drown your child or send milk squirting across the room.

~ Breasts can become infected. This means you can actually have bloody mild that baby shouldn't have. Not to mention how incredibly painful mastitis is. Good luck clearing up this infection, sometimes it forces a woman to stop breast feeding.

~Our culture is not structured for breast feeding mothers. Taking a baby to run errands can be a significant challenge. Imagine any public place, very few of them offer facilities for breast feeding mothers. If you are out and about, the most private place is probably sitting on a toilet in the public stall. How dreadful. Women need comfortable, clean, private areas to breastfeed. And when they can't find these facilities, they are relegated to covering the breast with a feeding blanket. This won't always appease offended others, so you have to juggle your baby, multi-task and take on the interest of others.

~Yes, breasts leak, and you can't control it. There are pads to put in your nursing bra, but sometimes they can become saturated and someone politely lets you know that you have wet spots on your breasts.

~Some babies have a hard time latching on or other problems. Sometimes they fall asleep in the middle of a feeding and your breasts are full of milk. You can wake the baby, but it may be disinterested. You can go pump that milk out, but baby will be hungry soon, and you can feed it a breast milk bottle, but your dealing with throwing off your breasts feeding clock and look forward to that engorgement again.

~ Some babies bite. Yes, they say, stop breastfeeding when the child gets teeth. What if teeth come early? My son had teeth at 4 months, breast feeding is recommended for approximately a year. I have scars on my nipples from his teeth marks. I also have stretched out areolas from his sucking. Babies aren't trying to keep your breast pretty.

~ Sometimes breasts don't make enough milk. My second son could not get milk off of one breast and preferred to feed on only one side. This is fine from a survival perspective, but it leaves a woman with one giant stretched out breast and a smaller breast. This is why it is recommended that you feed equally on each side.

Also, I think some women say they can't do it for a number of reasons, not just a body malfunctioning. I personally spent 3 weeks of trying to get my first son to latch on. Sometimes he could do it and at other times he acted hungry and didn't seem to know what to do. It's a very upsetting experience for a mother. Many women would have quit and said, I can't do it.

This is not a comprehensive list, there are many many other challenges. Back in the day, if a woman couldn't breast feed, she would use a wet nurse. I imagine this may go back millennia.

I hope this helps.