A quick google search made it more confusing.
https://www.quora.com/What-is-the-difference-between-trousers-and-pants
Quotes like:
"Both Trousers and Pants refer to the same garment, they both refer to an outer garment covering the body from waist to ankles, with a separate part for each leg. The difference between them lies in usage. The main difference between trousers and pants is that the word trouser is commonly used in the UK whereas the word pants is commonly used in America. It is also important to note that in the UK, pants refer to undergarments [later contradicted by somebody else]."
"I'm from the UK and I can tell you that there is a definite north south divide when it comes to pants. Up north we have always used pants to describe any outer garment that you wear on your bottom half with two legs, be it jeans, cargo or combat pants (see what I did there) peddle-pushers or trousers (formal slacks) etc. I have never even heard of underwear being called pants until recently, which is my reason for looking it up. This perplexes me somewhat. We call our underwear underpants for boys - general term for different types ie boxer's, briefs ect, and knickers for girls also which have different types. Simple. I think the difference lies in the word trousers. For me, this conjures up images of formal attire, like you would wear as a suit or with a jacket. So calling jeans your trousers would be wrong up north. We use pants as a generic word.I don't know whether southerners have to identify specific words in that case, because they can't use pants if it means something else!"
So it's a tribal thing.