I think it might have to do with the fact that the MWC is a way for these companies to show off their latest and greatest and as we all know Apple likes to be very secretive. There would be no point to spend the time and money to go to this conference to show off IOS devices that have already been released. The secrecy is what helps drive peoples intrigue into Apple products. However, maybe they just weren't invited ....Apple isn't exactly on the A list with some of these other companies after the recent lawsuits....
apple doesn't do trade shows because of how Steve Jobs was a control freak. He liked it better to hold keynote conferences on his terms by doing things this way Apple has full control over the event and gets 100% of the media's attention.