That was I was told in history lesson, too, but I'm not so sure that was all that important of a reason, otherwise it wouldn't have taken hundreds of years for people to adopt the new calendar. The drift is so slow that a week more or less over a period of 1,000 years is easily something that can slowly be adjusted to by farmers, politicians, and the like (unlike, say, the Roman calendar before Caesar reformed it which drifted by several days every year).