Originally Posted by namelessone
It's because of marketing.
Nadal was marketed as a "rebel" by Nike with the sleeveless shirts and capris in contrast the already established classic image of his main rival, Roger Federer(who had already dumped his locks by 2005). Youngsters will automatically go towards anyone that looks like he challenges the establishment somewhat, or at least one that looks "cool". Of course those guys also need to be succesful, not just have a "cool" look.
Nike hasn't let Nadal wear sleeveless shirts and capris since 2008. Nadal has worn simple tennis clothes since then.