They're the only ones with their crap together. They're the only faction able to manufacture weapons and armor. Not even the BOS can do that. Even though the institute can produce weapons and synths there's no way they could hold their own against the NCR army (even without the Rangers) They have Ghouls Super Mutants and Humans, oh and the Gunrunners.
The only possible faction with the power to rival or surpass the NCR would be Ronto, however this is only speculated from the lines Ashur spouts in The Pitt that assume he is calling out each of the factions for what they are best known for in the Wasteland.
" Citizens of The Pitt, workers of Downtown, traders of Uptown, and all the fierce souls who do what must be done! I bring you good news! We stand at the dawn of a new golden age. Where others merely survive, we thrive! Our industry is the envy of the Commonwealth! Our safety is the envy of the Capital Wastes! Our might is the envy of Ronto!..."