Time and distance. IF you were outside the "immediate" combat area, you had to be alerted, scrambled, assembled and directed to the Enemy. This took time.
If you can imagine the "average" squadron on readiness.....10 mins to scramble,,, they are scrambled, along with two other squadrons, not necessarily from the same airfield, they have to be assembled, and fed into the system...but the time element has moved the Enemy. Have they turned into the wing, away, or carried straight on?
Don't forget, at that time, Radar [RDF] did NOT work very well over land so the plotters/controllers could not see them, that was down to the Royal Observer Corps [who did a sterling job] BUT if there was significant amounts of cloud, they were snookered
I worked as a Radar op in the '60s, and the stuff was magic. I've also used WW11 stuff and THAT is witchcraft