Traditional explanation is that "that's how he really looked". There were sects at his times that did not cut their hair (Essenes, for example). Note that John the Baptist is also always depicted with long hair (but, unlike in Christ's iconography, also disheveled). Some people argue (* alas I cannot cite sources right now. Tom Wright? Craig Evans?) that based on the nature of his teachings, John the Baptist could in fact be a break-away Essene; a preacher of a more populist version of their general message. But either way, while secular standard was to have a short cut, there were sects and movements that did not cut their hair.

A less traditional interpretation is that Jesus was from Nazareth, and at some point the word "Nazarene" was mixed up with another word; namely "Nazarite" (yet another religious sect), and these guys were known to have long hair. So maybe it was an mis-attribution.

Or maybe Jesus was both from Nazareth and a Nazarite to begin with.