Well Micro-USB was standardized by the USB-IF in 2007 and the dock connector was first used in 2003, so I'm going to go with "linear time and a desire to be backwards compatible with the enormous ecosystem of products which provide them with a significant competitive advantage".
I mean, I can go to most hotels now and more or less count on the fact that even if I forgot to pack a charging cable, I can stick my phone on the alarm clock's dock connector. That's a big factor contributing to the low-friction iDevice ownership experience.
*edited to reflect the fact that the dock connector came in with the 3rd gen iPod in 2003, not the 1st gen in 2001.
I mean, I can go to most hotels now and more or less count on the fact that even if I forgot to pack a charging cable, I can stick my phone on the alarm clock's dock connector. That's a big factor contributing to the low-friction iDevice ownership experience.
*edited to reflect the fact that the dock connector came in with the 3rd gen iPod in 2003, not the 1st gen in 2001.