I revisited the sensor detection ranges with a fine tooth comb over in the "how do you pull off a raid" thread. Sensor detection, according to the current rules in Strategic Operations, is pretty abysmal. Sensors won't detect spacecraft more than say 1/3 the distance between Terra and Luna. Interception for space battles is just about technologically impossible except at the beginning and terminus of transits. If you don't catch dropships either at the planet or at their jump point, you basically can't catch them. Except of course for the "one in a million" chance of transits happening to just randomly intersecting each other with such proximity that they can angle their vectors to make a very high speed pass on each other within weapons range.
And that's where most interceptions actually happen in universe. An invading force is pretty easy to track and tell where their end point is. It's not hard to put yourself in a position to intercept at lunar range to the planet for someone coming in from either standard Jump Point. High orbit interception is relatively easy compared to that.
Consider how difficult it is to change your vector when you are braking your speed, especially if you are trying to hit a relatively small window that you know your enemy will hit.
I honestly think that the reason aerospace forces don't fill up that much is because the best people to be piloting ASF, are usually good pilots for 'Mechs. The combination of spatial awareness, ego, and reaction times are vital to a Mech pilot, but even more so with ASF. But the 'Mech is more popular, so more people gravitate to piloting them than ASFs.
Think about it this way, if you have a really good sprinter, and he can catch a ball pretty decently, will he push to being an olympic sprinter or a professional football player?
So, too, ASF pilots are relatively rare when compared to 'Mech pilots. And it actually takes more skill to pilot an ASF than a 'Mech. You have to have a greater 3D awareness in AS combat than on the ground. You have to be aware of power curves, courses, and fuel consumption that isn't as prevalent (if at all) in a 'Mech combat.
The final point is that, as has been learned throughout the 20th century, aero assets cannot control objectives to win wars. At best, they can completely destroy the supportive assets that run a war machine and allow for that machine to travel. They can cut down and reduce forces in transit, but that's about it. To take and hold a facility requires ground forces. If you want to scorch the earth of the Draconis March, to make sure they can't produce the materiel and manpower to fight you, it's possible, but you can expect the same thing to happen to you when your enemy gets a chance.