Apple likes to set trends which means doing away with the past. The headphone jack is a 19th century invention and Apple feels that the time has come to make way for new tech as it has always done.
Add to the fact that they obsess over making their devices beautifully engineered by making them slimmer and lighter to increase portability. The more portable your hardware is, the more you are likely to use it and recommend it to others. Portability helps Apple sell more devices. This has been Apple's focus since Job's return.
A few examples:
1. They were one of the first laptop makers to do away with integrated CD/DVD drives because they saw that fewer and fewer people were using them. The tradeoff here is obvious: they figured that Internet speeds would improve to the point that it would eventually replace 4.7GB DVDs as the standard means of distributing software for most of their customers (read: people with high disposable income). This has happened.
2. Apple was the first to ship a mainstream device (iMac) that supported the USB standard (which they didn't invent). USB adoption helped obsolete older tech: serial, parallel and PS/2 ports.
https://en.wikipedia.org/wiki/USB#History
3. The iPhone obsoleted keyboard-centric phone designs from BlackBerry, Nokia, Samsung etc which were widespread prior to 2007 to a more natural touch-centric interface. Apple had previously launched the failed Newton device but this time they made several complementary investments in the user experience that made them feel confident that touch interfaces had matured enough to become mainstream. Remember that the first iPhone didn't even have copy&paste, regardless, their bet against old tech paid off :).