Can someone make this clear for me by explaining, why 4s comes before 3d, when writing the electronic configuration of elements? For example, chromium, its electron configuration is, 1s2 2s2 2p6 3s2 3p6 4s1 3d5. Why 4s isn't after 3d?

Because before scandium, the #4s# orbitals were lower in energy than the #3d# orbitals. For calcium, that is the case, and the #3d# orbitals are known to not be occupied, so one wrote #4s# after #3p#:

#[Ne]3s^2 3p^6 4s^2#

At scandium and past, the #3d# orbitals dip below the #4s# in energy, but some sloppy extensions of the Aufbau principle you may see out there (that have a VERY specific, winding path through orbitals...) do not account for that, and out of "habit", I suppose, we tacked on #3d# after the #4s# regardless...

You can read more about that here.

CONSIDERATIONS REGARDING ORBITAL ENERGIES

It's just become a bad "habit" to write #4s# before #3d#. It really should be with the forethought of which orbital is next highest in energy, and that one goes next in the configuration. Hence, you really should write, for the first-row transition metals,

#bb([Ar]3d^? 4s^?)#

This indicates that the #4s# orbital is higher in energy. All data for orbital potential energies I reference in this answer are found here.

ORBITAL POTENTIAL ENERGIES

See how the #3d# orbitals are lower in energy than the #4s# for the first-row transition metals here:

And you can further see how the Aufbau principle fails for the heavier transition metals, in that the #(n-1)d# and #ns# orbital potential energies criss-cross rather unpredictably:

Every single resource I have checked had #"4s"^1# at the end, after #"3d"^5#.

As it turns out, for the first row of transition metals, the 4s sublevel has higher energy than the 3d sublevel, which is contrary to what we would expect from an Aufbau diagram. So the electron configuration for chromium is written in order of increasing energy, with the 4s sublevel having the highest energy and written at the end.

Thank you to Truong-Son N. :)