Music changed because of money. Club owners had the choice of paying for big bands with 20 people being paid musician union wages or four kids getting a set amount of money. The guitar groups added in horns and strings after they were successful and could afford it.
Big bands were always economically problematic. Part of the problem was an FDR wartime “nightclub tax” which contributed to the demise of the big bands. Hollywood and the recording industry could provide the economies of scale to justify big bands, but popular tastes changed to smaller ensembles. I bet the Stones got more per performance than any of the big bands ever did.