You already had a player to compare it to, you had a device that had limitations that you wanted improved upon, it was easy to see why a better designed version would be an improvement.
It is harder with a new category of product. It was hard for me to see the appeal of the iPad, after all I already had a iPhone, why would I need a big one, but then I tried it and it was pretty bad ass, and once I owned one (which I needed for development purposes) it quickly became a huge part of my daily electronics usage and replaced my laptop for the vast majority of non development tasks.
I suspect this is similar, if you had a pebble and you were using it you would probably look a a device like this and instantly be able to see the ways it is better than the one you have, but since it is a type of device that nobody really has it is harder to understand the ways in which you would use it. This is not just a problem for apple but also for any other smart watch developer.