is not rebellious to have long hair in wizarding world, and less among old families the were into the pure blood ideology
It’s a suuuper common headcanon that Sirius kept his hair long to piss off Walburga. It’s not a stretch, given that rebellion is like 90% of his personality. Maybe his parents have a distaste for long hair despite common pureblood fashions, or maybe just for the way he wears it!