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I suspect it was a marketing decision. The consistency of his clothing choice, helps to reinforce the brand (in this case Steve and Apple) in the public mind via his portrayal in media. For those who are aware he always chose the same clothes it's merely an idiosyncratic trait, however for the wider public who are only dimly aware of leading computer figures, it aids recognition ("ahh that's that guy, who runs Apple") above the background noise. There's a bit of an analogy with brand connection/consistency in choosing similar product names eg - iMac, iPod, iPhone. Consistency, helps cross-recognition and positive association. It's all a carefully crafted narrative for the public mind.