Milton uses the allusion to Atlas because this is an epic poem, and as a result he is trying to follow convention. He uses Atlas in particular because much like the fallen, Atlas has been punished for going against the Gods. While Beelzebub isn't carrying a tangible weight like Atlas does, he is feeling the pressure from the minions in Hell that their future lies in his hand, his words. Beelzebub feels that he is responsible for the future of the citizens of Hell.
-Erik Scott