It is a Shinra PR approved fashion statement

Feels nice like to him

His hair grows fast and he’s given up

Being petty about a comment Hojo once made about his hair

So people can see him easier on the battlefield

For Hojo to get maximum samples

Genesis or Angeal said his hair looked nice longer 5 years ago

Doesn’t like haircuts

It’s a stim toy

He was trying to keep waist length hair and failing

Other ideas

It wasn’t that long before he died. Cloud has shit memory