In my view..
If He would present in a molecular form then i would blame to the formation of energy band due to overlapping of molecular orbits.
But this is not the case as He present in elemental form.
So I think that
This happens due to sheilding effect of electron..
As sheilding effect of electron varies with the orbital in which it present.
Suppose a situation of an excited He atom,
first elecron is present in n=1, l=0 (1s) and second one in the n=2,l=1(2p) , so the energy level of all shells will be slightly different from the energy levels of He in the ground state.
This is only one of the excited state ...there are many other possible state.....so now you can think about it.
This change is main reason that the no. Of spectral lines are greater in case if He.
Or see it in this way suppose there is a H atom ...suppose you can observe 'n' energy levels.
Now in He atom there are two electrons so there will be
nC
2 (= n(n-1)/2 )observable energy levels.
It seems logically.. I didn't read it anywhere ..so if anyone knows that my point of view is correct or wrong.....so please tell me....