Fashion never mattered in the other categories that Apple entered to the extent that it matters with watches. The looks are a key feature in the watch category in a way that looks never where (at first) when Apple entered key markets like music players, phones, computers, etc.

If Apple entered the quantified self category (not smartwatch), they would've just launched with a single device as they did with the iPhone/iPod. Those devices are not much in the way of statement pieces, outside of saying "I'm a nerd!" or "I'm an early adopter!"

With Apple Watch, they're entering a category in which fashion and looks are a key feature. They know from the iPod mini and nano what it takes to become a fashion statement piece - and its the variety of colors/styles that make that possible. Everyone having the one single device says less.

Think about the white iPhone and the gold iPhones - their looks made them more sought after, they let people stand out. The iPhone would never be the fashion statement it has become without that variety that makes it feel/look more personal. The color you choose says something about you. Apple was acutely aware of this as far back as the rainbow iMacs in 1999. Watch Steve's keynote intro of these - he speaks at length about how much color selection says about people.

Watches are the first category they're entering where fashion is such a key feature:

Apple II / Macintosh helped establish a consumer market category for the first time. Everything was ugly and niche.

iPod just replaced portable CD players that all looked/worked the same. They were not fashionable accessories before iPod, the music you put in them was your statement-maker.

iPhone replaced (crappy) dumbphones that few really liked, and redefined the smartphone category which had little differentiation w/ small screens and physical keyboards. Razr and Blackberry were closest to being any kind of status/fashion icon, but barely. Style/status-wise, Apple instantly blew them away - except in the corporate world - that took longer, but they did it.

iPad created a new category of personal computing. There was nothing to beat here - they had to establish new behavior among consumers.

Apple Watch enters a established market with a history over one hundred years of fashion/functional evolution of an iconic and personal fashion accessory. The device they are replacing is embedded in our culture, our families (nice watches get passed on to new generations), and our lives in a way that none of the above device categories ever were.

Watches are more of a fashion statement than anything else, especially in the age of smartphones. If they launched with a one style fits all watch, it would be tough to win over people who care about making a statement with their image. iPhone replaced inferior devices. Apple Watch replaces personal statement pieces.