Because it's not going to have an electron configuration of 1s^2 2s^2 2p^6 3s^2 3p^6 4s^2 4p^6 in the ground state... iron has 3d orbitals available... the energy ordering of the 4s and 3d may be lacking consistency among some textbooks, but certainly not the 4p and 3d.
overbrace(2)^(1s^2), overbrace(8)^(2s^2 2p^6), overbrace(14)^(3s^2 3p^6 3d^6), overbrace(2)^(4s^2 4p^0)
K, " "L, " "M, " "N
Iron cannot skip the 3d orbitals and use 4p orbitals. We know already that the 4p orbitals are higher in energy than the 3d orbitals, by virtue of the 4p having a higher l than the 4s, and the 4s being at least CLOSE in energy to the 3d.
Hence, the 3d orbitals are occupied (and NOT the 4p!), as expected by the Aufbau principle (iron is not some weird exception here!).
The 3d orbitals are in the so-called "M shell", so in "KLM" notation (which is not particularly clear, but is nevertheless used for X-ray experiments), one would write color(blue)(2,8,14,2).
overbrace(2)^(1s^2), overbrace(8)^(2s^2 2p^6), overbrace(14)^(3s^2 3p^6 3d^6), overbrace(2)^(4s^2 4p^0)
K, " "L, " "M, " "N
