Why is it so hard for some to set and maintain boundaries?

Because they were never taught to do so. Coming from a family who didn’t know how to set them you are somewhat of an enigma. Congrats. But try not to judge others.

I think it depends on how much you feel you have to lose in any particular relationship. With my mom I guess some people might say we were enmeshed. I personally can't stand these psycho babble catch phrases. Truthfully after losing my dad to cancer at a young age I clung to my mom, afraid of losing her too. That relationship established itself at a young age for me and lasted all through my adult life. To change it would have been traumatic for me even though it probably would have been good for me to do so. So no boundaries there. I felt I had too much to lose.

But I have some really selfish, mean, narcissistic siblings who proved themselves to not be worthy of my time or effort when my mom got old and sick and they were nowhere to be found. After my mom's death I put up solid boundaries with them cause I feel I have nothing to be gained by letting them into my life and a whole lot of possible hurt if I did.

So, as I said in the beginning of my post. Boundaries are dependent on each individual relationship and what you stand to gain or lose by having them or not having them.

'... a lifetime of conditioning by her that she was always to be kept happy or there would be heck to pay.' You have said it yourself. It is incredibly hard to get away from that ingrained mindset, brainwashing, whatever you call it. It has taken me months of counselling to accept that I am not responsible for my - unpleasable - mother's character defects and life choices, but I still feel pangs of guilt that I'm not doing enough for her.

I say, we are all different in the caregiver role. I based all my decisions on 'what decision can I (my heart) live with'. I used the same decision making for all of my immediate family. I'm pretty much bound by helping my family first and foremost. I don't really set any boundaries, per se. I do what I can when I can and try to cover what I can't do with other means.

Narcissists train their children not to have boundries. To be doormats. They aren't separate people, but extensions of the narcissist. They can control them for life like puppets.
My husb is afraid of his mom. She verbally attacks the other wives. I havent seen it, but saw how they act around her. Not one son would stand up to her. One wife keeps her head down and says nothing at family visits and meals. The whole 4hr visit. I refuse to do that. I havent seen her in years or go to family functions. Im not doing it. She insulted me once in front of the other wives and grandkids. It was just my turn. She made sure her husb and sons were out of the room and couldn't hear. All 3 sons told me get over it. That i have to endure it. She's like that. I said no.

Husb was supposed to go to lunch with his mom, and then do something for me in the afternoon I needed help taking it down. He knew about it days before the lunch. It needed to be done that day. I had to drop it off at a persons house to be fixed. I thought he's gonna tell his mom in passing, and she will make sure he doesnt come home. Sure enough she planned his whole day. Driving her all over. It was to late to do it when he got home. And I had to apologize to the person I couldnt make it. It was an appt. He tried to start a fight over it, and I said nothing. I didnt want a fight for a week. So I decided side step all of it. His mom would have loved knowing she caused a big fight. I'm afraid since he's retired, she's going to start making him her servant. She is that type.

It's also weird when he talks to her, he has to go in another room or outside. I never did that when talking to my parents. I had nothing to hide. Sometimes after being with her, he'd start a fight. I think she picks his brain and sees where she can poke. Her sons cannot stand up to her. Not 1. Even the grandkids ignore her at family get together. They disappear like we are not there. They scarf dinner in 2 minutes, get up and leave the table in the middle of the meal and hide in their rooms. I've never seen anything like it. This happened every Thanksgiving/Christmas.

She flipped out one year that her 6 yr old granddaughter played with a toy my husb bought her for Christmas. She demanded the child stop playing with it, put it away, and play with the sewing machine she bought her. The child wasn't interested. Not one of her sons said let her play. It's Christmas. She made a huge stink over it. Was screaming and carrying on for 5 minutes over it. I thought Christmas toys were to play with? She was that jealous the child chose my husb toy over hers. She made a big production of showing her granddaughter sewing some fabric. The kid wasn't interested in sewing. Not at that moment. They all walk on eggshells for her.
It will never change. So you can't win that battle. I tried. It's not worth it. You have to side step the arguments because she wins then.

I suggest watching crappy childhood fairy on youtube. And how to spot a narcissist and how to deal with a narcissist. And how not be be narcissistic supply on youtube.