Here are some reasons why Sudoku requires skill rather than luck.
You don't need luck when you have logic.
Sudoku is a logic puzzle that requires deductive reasoning and the process of elimination to fill the holes in the grid. So you don't need luck, just a clear view of the grid, the ability to concentrate and the determination to keep going even when things are difficult. The concept of luck has nothing to do with the hard work represented by slaving away and working your mind on the sudoku grid.
Guesswork is counterproductive.
Sudoku players are sometimes tempted to guess. If you can't figure out which number to place in an empty space, it seems like a good idea to guess what the right number is and move on. However, guessing can lead you away from the solution of the sudoku, and if you guess wrong, and that wrong number becomes the basis for other incorrect assumptions about the puzzle, you may create a series of confusing errors that lead you to abandon the puzzle in frustration. In evil sudoku mode, guessing wrong will add another error, bringing you closer to defeat.
Hoping for luck can hinder your progress as a sudoku player.
If you try to guess or look for lucky shortcuts, you are only hindering the development of your sudoku solving skills. Learning to play Sudoku is a skill that does not rely on luck or serendipity. The best players know the fundamentals of the game, they know how to analyze a sudoku grid. They also know how to apply a rigorous vision and thought process to sudoku solving techniques. If you take the time to practice solving sudoku puzzles and learn the basic techniques, you will be far better off than someone who thinks sudoku is about luck.