First, printing two different versions of the calendar, with no way of knowing in advance how many to print of each one, is not financially feasible. The calendar already teeters on the edge of whether or not it makes or loses money, depending on how many I order, due to the fact that by mid-March they will be dramatically reduced in value to most customers.
Second, I write the jokes for the bottom of the calendar to fit the amount of space that each month has before and after the days. If I were to change to a Monday-first version, it would shift all of those spaces one over, which means it wouldn't fit the characters who are telling the jokes without me rewriting them to maybe exclude one character or add another. That is far too much work for very little return, and that's why there won't be a digital version regardless of the printing issue.
And since I know the next question is why I make it the American way in the first place when the rest of the world does it differently: I sell something like five times as much stuff in the US as I do the entire rest of the world put together. There may be more people in the world who start their week on Monday, but there are more paying customers who start theirs on a Sunday.