I.E. is one of the largest, because it's Microsoft, and it's what's on a clean install of a computer, when a person buys it.
I'm quite certain it's market share is only due to this fact, not because people prefer it as a browser of others, they just don't know the other browsers.
I'm in agreeance that I find Opera to be the fastest of the browsers, however I wouldn't call it the most simple to set up, or use..
It's probably the most raw browser out there, which allows it to do a wide variety of things through it's features, however for the average user, they just want to be able to plug and play.
They are going to likely keep using I.E., or will use Firefox as it's one of the most advertised out there pusing it's blocking of pop ups, which everyone hates, so they are often happy to try it out.