Lots of reasons:
I had good care (2 midwives throughout as one was a student & I was being induced). This was essential during transition when I started to feel overwhelmed by the pain.
I was induced, had G&A and one shot of pethidine. 10 hours or so in labour.
I have a high tolerance of pain - this is a genetic thing, not something that makes me 'better' than anyone else. That's just genetic luck!
I wasn't scared - I really do think that helps. I had all sorts of problems the week before DS was born, which actually helped as it made me face up to the worst case scenarios. I freaked out, had a massive tantrum and then was able to accept being induced and what the possible outcomes of that might mean. Hence, my fear of childbirth was actually lessened and I think this helped me cope with the pain.
A relatively quick, straightforward labour and delivery. If it had gone on for hours and hours I'm sure I would have needed more pain relief.
I can't stand the thought of the needle in my back.
I hate hospitals and interventions and the thought of being incapacitated by an epidural is a strong motivation for me not to have one.
I was able to remain active and upright throughout labour and delivery. I'm pretty sure being flat on my back would have made the pain intolerable - I was very vocal and clear about NOT being confined to bed and luckily, despite being on a drip, my midwives supported me. Though, tbh, they didn't really have a choice... 
So, in summary it comes down to good care, whether you are able to tolerate pain, remaining active & feeling informed/in control of what's happening to you. The latter point in particular is partly down to you but also partly down to luck - and so for this reason I feel vert strongly that ALL women should have access to whatever pain relief they want or need. It bloody hurts and it does sometimes feel like HCP forget that every birth is different and every woman is different - and just because one woman can cope with a straightforward labour, doesn't mean that another can or should.