When the Romans laughed at the Celts for wearing trousers, it was because they had not visited Britain. Trousers were considered uncivilised, but when the Romans arrived in Britain they discovered that the weather was not very pleasant.
Trousers are the best clothes for keeping warm, particulaly in the fields or on the road. So they were adopted very quickly.
Within the Roman army they were used by the cavalry, were often made of leather.
For more information on trousers see Celtic Clothing