Another thread commented on the sad decline of the brotherhood (west coast BoS that is) and I had this thought.
The Brotherhood of Steel is a metaphor for reliance on Technology.
Think about it, they were the sole source of powered armor at one point, the only people with the biggest of the big guns.
But as time's worn on they've declined in power, even as civilizations rooted in the post-war wastes flourished.
The BoS originated with the US military at West-tek, NCR originated with one small town in the middle of the desert. The BoS has had their fortune steadily decline over time, while the NCR grew from a town to a region to an empire.
The NCR creates (the sharecropper farms, ect) the BoS re-appropriates.
The Brotherhood and the Enclave both represent, metaphorically, the fortunes and importance of the two bastions of prewar US pride and power: the Military and the Federal Government. The Enclave turned evil and actively destructive when they lost control, the brotherhood stood for reclaiming the past but gradually as the past has lost importance, they too are relegated to a minor role.