Also, I think the weight of phone is very important so anything to shave even a few micrograms off it could be one of the reasons (Read: Why Steve jobs tossed the first iPad in an aquarium [1])
Lastly, i think it is also because of what you said, especially with the new closed walled phones where the device manufacturer (read Apple) would like to seal you into a contract with the phone company and phone.
[1] https://my.news.yahoo.com/steve-jobs-dropped-first-ipod-1521...
By making all our connectors different, we have to keep buying stuff. Consider that I can't take my SIM card out of my phone then put it in a different phone. That means I have to buy a new SIM card, if I want a new phone. That means sales of SIM cards.
Consider that we have both Firewire and USB, and that USB has four different kinds of connectors. It moves product!