The Doom of the Noldor

[Kuruharan]

I wonder why was it so important to Morgoth to prevent the joining of Forces between Maedhros and Fingon.

Because "divide and conquer" is one of the fundamental principles of warfare. It's so much easier to fight two small seperated forces than one united stronger force. Especially if you have a central position between the two.
Even assuming that Morgoth would have won anyway, it's always best to take the path of least resistance.

I have always wondered why didn't Fingon called for the Help of Orodreth.

Did we talk about that in another thread? The topic seems familiar but I can't quite place it. Or did I imagine it?