It was a new, simpler way to do things that people were already doing. Marketing was part of it, but it was the streamlined UI/UX that appealed to most. It's more streamlined than Symbian at that time.

Underneath it all, Apple had been laying the groundwork for a one-stop shop for music, video and other media as well as it was a place where you could seamlessly and quickly add/remove stuff from your phone.

Simply stated, CTRL+C and CTRL+V (or even drag and drop sometimes) for some folks was too much for an USB mass storage device for some folks. Apple had found a way to lower the entry level for "smartphones" (loosely applied here) from savvy to just "if you have this much in your pocket, congrats... you're a smartphone owner - Apple"

I think it's more of a phenomena that iOS is catching on in Japan. What they call usable is quite different than say Europe and/or North America. But the simplification for something that is supposed to be a convergence of media, contacts, phone calls, gaming, et al and as it stands, iPhone was doing it in a simpler manner than most. Did others do it even more simple, yes they did.

But marketing said Apple's efforts were simpler and people believed it "just worked". There are a ton of other factors, but a streamlined, uniform UI/UX was what got the ball rolling, but the media, software, OS and other parts all contributed to that success - distribution also had to be in place, Nokia dismantled their distribution (online stores gone, stores in the North American market gone... I have to buy via Amazon or some other 3rd party). And ultimately... Apple walked in to the carriers and successfully made them deal with them on their terms. Nokia was not successful at this outside of Europe.

That's my take. Feel free to dissect, but each point above is where Nokia is failing or have failed.