I had such an awful time trying to BF DC 1 and 2. I tried, I tried until I was sobbing my heart out with frustration, pain and complete and utter desperation to make it work. You're practically lectured throughout the entire pregnancy about how amazing Breastfeeding is and made to feel like if you don't, you will be failing your child. It's a massive pressure and I felt it really ruined the early days with my first two DC because I spent so long panicking, worrying, stressing and upsetting myself over how I could get it to work.
With both of them I struggled with latch. DC2 point blank did not want to bloody latch. She'd throw a massive tantrum every time I tried. It would be latch, pull off and scream, latch, pull off and scream. My nipples were bleeding. I bought a pump and spent about an hour getting a measly 2oz out. DC1 seemed to have colic from birth, he screamed and constantly seemed to be in some kind of discomfort. I still don't know what it was because you get told colic is impossible when Breastfeeding but he had all of the symptoms... It was horrible and I felt so helpless. Switched them both to formula and it was such a bloody relief. DC1 colic symptoms cleared up, DC2 didn't tantrum every feed plus DH could now share the night feeds.
DC3 was, for whatever unknown reason, completely different. She latched with great ease, never had cracked nipples, she breastfed wonderfully and for a year until she decided she didn't want to anymore.
So, those are my reasons. It wasn't for lack of trying with first two DC. As for people who FF without trying.. Well my DM was one of them. She tried once and hated the sensation of it, hated how it all felt and thought it was a bit 'creepy'. She just didn't find it came naturally to her, it made her feel like a cow. I can kind of understand that view. I guess simply not wanting to is a reason in itself. In the end it doesn't really seem to matter anyway. I have no health issues, DC are all on the same levels despite one being BF and two being FF. It becomes a total none issue when they're past the milk stage.