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It's because of the Aufbau Principle which states that electrons will always fill up oribtals with lower energy first. The 4s orbital is slightly closer to the nucleus than the 3d orbital and therefore is considered a lower energy orbital.That's why electrons fill up the 4s orbital before the 3d one.
I agree with the previous answer and would just like to add that once the 3d subshell is actually filled, it then becomes lower in energy than the 4s one. This is the reason why we usually write 3d before 4s for electron configurations even though on the periodic table itself, 4s comes before 3d. It's just because 3d stops being higher in energy than 4s once it's filled up. Until that, though, 4s is lower. I hope that helps!
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