Because there's already another 6 on the same main diagonal. This is an "X Sudoku" puzzle, which means that each main diagonal must have all 9 different numbers.
https://imgur.com/a/Dfxf9CJ (before picking 2 / 6 / 7.)
Its solver is interesting in showing the individual steps.
For the same puzzle, https://www.sudokuwiki.org/sudokux.aspx?bd=06039000804900150...