Is there something wrong with the back button? All the cancel button would be is a link back to the post, which can also be achieved by simply hitting the back button.
The difference between the main site and mobile is on the main site, the editing screen is loaded inline with the page, while on mobile editing actually takes you to a new page. Therefore, it's far easier on the main site to just reload the content inline rather than forcing users to reload the page. On mobile, it doesn't matter. You have to reload the page no matter what to get back to the regular content.
I do believe the main site used to go to a separate page as well, and the cancel button was implemented when they started loading it inline, but I'm not 100% sure. Not to mention, heading for the back button on a desktop computer requires slightly more effort than on a mobile device, especially something small like a phone.
The user interface on mobile devices is very different. The back button is often used to cancel edits (I know I use it whenever I cancel things), so using the back button makes sense. Browsing through my phone just now, I couldn't even find any applications I have that provide any sort of "cancel" option when editing things, only the use of my back button. Why should the Stack Exchange network be any different?
The only real reason I can think of for adding this is consistency across the full and mobile versions of the site, unless I'm missing something.