why are s,p,d,f block written is small letter

dear friend its just a method of representing the various orbitals.
actually it is the azimuthal quantum number
for l=0 we have s orbital
l=1 p orbital
l=2 d orbital
l=3 f orbital
and so on
similarly principal quantum number is represented by K,L,M etc
also you can see that PQN(principal quantum number) is represented by capital letter so for convenience AQN(azimuthal quantum number) is given small letter