To be fair OEMs look at what the average consumer wants and build phones based on that. The proportion of consumers wanting more than 16gb on their phone has to be insanely small and given that Microsoft is going after people getting their first smartphone 16gb is just plain unnecessary. Once the majority is asking for more than 16gb it will become more common place.
It's a fairly common misconception of the tech-minded community to think that what they want from a phone is what every one else does/should want. The vast majority of people don't get a mobile to listen to music or watch moves, they get it to make phone calls and send messages and maybe go on Facebook.
While I agree that more storage is going to attract a small number of customers to a phone, for the vast majority it doesn't make a whole lot of difference especially given the increased costs. How many people on the street are going to see and 8gb phone and think, ahh but if I spend ?100 more I can get 16gb, or ?200 more for 32gb.