I will briefly explain.
Basically, you have a Four Factions fighting over Energy, Water and other resources that will make or break them.
You have a Republic that has grown so large and Corrupt that it can't even pretend to control it's own borders, while it sends it's Army off on Far Away Adventures and entanglements while it's borders remain wide open.
In The East, you have a Vast Slaver Army who is a throw back to earlier more brutal times and doesn't allow for modern notions of right and wrong to stand in their way, led by a central figure who uses a brutal religion to indoctrinate and enslave the people he conquers or wipes out.
While in Vegas itself you have a Autocratic Technocrat who wants all the power and means at his finger tips and everyone to be his puppet.
Finally, you have the people of Vegas, a Band of Tribals, ingrates and Gamblers who couldn't agree on if it was raining even if they were soaking wet.
Now in Fallout 4 we have infiltrators who pretend to be your friend but will kill you as soon as look at you and The Invading Civilizing Forces options are rapidly demising to Nuking the Entire Site from orbit just be sure.
...not to mention a whole host of drug raged roadside cutthroats to keep you company.
Siding with the NRC was an easy call for me.
Both the gladiator movie guys and the cop-bots rubbed me the wrong way from the start.