Maybe it's because they've gotten so big, they've lost their ability to be flexible and adapt quickly.
Well, how do you explain companies like Google? I mean it is arguable that their business plan has always been expansive from the birth of the company, but they have managed to continuously push forward no matter the failure or success of the product/service. Even Microsoft with Windows 8 was looking to change their overall design, and it ultimately didn't fair to well, but Windows 10 (although its too early to tell) is being hailed as a great mix of old and new in terms of interface. I feel like Steve Jobs started something brilliant, and Apple just kind of rode his creation. Now they are in a position that if the magic of the iPhone branded products wears off they are pretty much done for.
I think for Google, they have always had a history of expansion and innovation in mind. Microsoft knows they need to change or risk becoming irrelevant. I think Apple is turning into the Microsoft of the early 2000s, slowly losing their edge but still too big to see it yet.