Sunday is considered the first day of the week in the United States and countries that followed the Greco-Roman tradition.
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The week numbering system was established in ISO 2105, 8601 is a later standard that supersedes it.
Monday is (typically) the first day of the working week, and as such is (typically) referred to as the first day of the week in business and the industry. Furthermore several spoken languages consider Monday the start of the week.
And it is so only in the USA, Canada, and Mexico.
If all choices are arbitrairy, it is not logical to follow a small minority.