The clone/stormtroopers' armor is just a battle uniform, and those change over time, due to changing tastes and technology. Compare a US Army ACU of today to the BDUs of twenty years ago; different camouflage pattern, design, tactical vests, kevlar, boots, etc. It makes sense that stormtrooper armor would evolve quickly, especially given the rapidly changing military and political climate of the late Old Republic and the Empire.

Vader's outfit, on the other hand, is his sole outward identity. It's his face to the universe, and it should remain the same throughout his life -- aside from the occasional repairwork, upgrade to the internal gadgetry, and polishing.

Though I do wonder why Palpatine had such a suit handy, and whether its appearance had any meaning. It looks a little bit like Warb Null, the Naddist enforcer on Onderon. But Palpatine's Sith traditions, as a whole, seem divorced from the ancient Sith; he goes so far as to insult the Lords on Korriban near the end of his life. Vader was more in touch with the original Sith, as there was a throne prepared for him on Korriban.

Of course, if Vader's outfit screamed "DARK LORD OF THE SITH," everyone in the Galaxy would soon know the true source of his power and patronage. Maybe he just wore a standard issue Sav-a-Life Suit(TM) from Republic MedTek(TM).