Tradition is mostly the reason, but there is some data to consider. The bible declares the sabbath to be the last day of the week. From other sources, it appears that at the time of Christ the sabbath corresponds with our Saturday. The Russian word for Tuesday (вторник) means second, indicating a history of starting the week with Monday. Other languages follow this as well.
To set things straight, ISO created the standard, ISO-8601, that among other things, set the first day of the week to be Monday.