The reason for this term was to marginalize Christianity and to indicate that it came from a small area, a religion of only local significance.
In the same manner, Jesus was called "of Nazareth" or "the Nazarene" in this context (another emphasis on geographical origin and the indication of local significance) instead of "Christ" (derived from the Greek word "anointed").
Over time, the originally factual designation absorbed a pejorative meaning, which, despite its expansion throughout the Roman Empire, still referred to it as a local religion.
The use of these terms in late antiquity was generally meant to weaken the religion, gaining an advantage just by the naming, before making any arguments.
Stoic philosopher Epictetus uses the term Galilean in his Discourses (c. 108 AD): And is it possible that any one should be thus disposed towards these things from madness, and the Galileans from mere habit; yet that no one should be able to learn, from reason and demonstration, that God made all things in the world, and made the whole world itself unrestrained and perfect, and all its parts for the use of the whole?