Engine idle fuel flows are predicated on being able to get go-around power in a certain minimal time interval. Whilst you could quite happily tweak engines to get a lower idle speed, when you needed rapid power, it would be no good sitting there with a rotated aircraft and terrain rapidly approaching whilst the engines happily burbled away thinking about increasing N1 as the fuel flow appeared to be increasing. Remember the AA757 Cali. You must always be able to get high power quickly, so what decides idle speed and fuel flow is that engine types' response to power increases at those lower power settings.