Why did the INF Treaty focus specifically on ground-based missiles?

Factual Questions
1

Why was intermediate range (500-5500km) a big deal over ICBMs and tactical BMs?

Why were ground-launched missiles singled out over sea or air-launched ones?

Why include cruise missiles since they would be less useful for a nuclear engagement than ballistic missiles? Why prohibit a 1000 km-range cruise missile launched from a TEL but not a fighter?
From the US point of view, it makes a lot of sense. Ground-based intermediate range ballistic missiles were one of the few areas where the Soviets had some kind of advantage by the 70s and 80s. Why did the Soviets agree to it, though?

2

WAG. The short flight time from launch to impact would give almost no time to retalliate, rendering MAD useless and increasing the attractiveness of a preemptive strike.

3

The simple answer is that it was a US advantage and in the political and economic climate of the era, they got their way.