I agree short hair did look better. The reason was that during the escape scene with the tribal members Jackie fell from a tree while doing a stunt. He cracked his head and almost died.
Apparetly he had/has a small hole in his head from that accident and grows his hair out longer to hide it.
Indeed as thewagon34 mentioned part of the reason was to hide the hole in his head better. However Jackie has also said that (i can't remember who perhaps Willie chan or someone else) someone told him not to cut his hair short as he got injured when he did, and that it was bad luck. Clearly in the same vein of superstition as when meals on wheels was changed to wheels on meals as 2 movies starting with M had flopped before it.
Oh and i really don't think the short hair did suit him, it made him look kinda childish and just generally didn't fit him as well i think.