haggul - 12:45 pm on Aug 31, 2012 (gmt 0)
Mobile - well unless your site is fully mobile optimised I doubt many people want to complete your checkout process on their 3 inch screen. Also a lot of people will research from their mobile, or read your email newsletters from their mobile, and then buy on another device later which is a little easier to form fill from.
Other reasons - maybe parts of your site don't work so well on other browsers / devices? Test as much as you can.
Could be finer reasons than that - maybe your products attract more Mac users - arty / wealthy types - I know a horrible generalisation but on a fine art site I built it's 100% true!