Of course a long time ago film was only blue- and uv-sensitive. Then as technology marched on, the manufacturers managed to make it sensitive to longer wavelengths.
My first thought as to the reason the film only responded to blue is that shorter wavelengths are higher-energy. So it's just that the film is relatively more sensitive to the blue photons. But if that was the case, it wouldn't really be possible to obtain panchromatic film except by making the film LESS sensitive to blue so the other wavelengths can catch up.
So please explain to me how panchromatic film 'works' and why early films were blue-sensitive. Feel free to get technical.