There is none Elerium beam because beams are the first weapon you get and if you carefully look at the entire line, beams are the most balanced and at Doom Ray the most powerful.
Back in Alpha through Beta, beams were the best weapon hands down. With the addition of proto type weapons and more balancing the other weapons came online. Beams have a great accuracy and great range.
I think the choice was made to give other weapon types proto/resource status as that encourages diversity to weapon load-outs.