If they can call us out of the blue, we should be able to answer back. Saying "no" over the radio clears up the mission from your map, keeps them from being (as) pissed about you blowing them off, and might even give you a different way to appease them instead? Or at the very least, frees you up for a different enclave's call.
This keeps people from having to say "no" in person, saves players from boring drives, and adds immersion to the world by having communications be less of a one-way street. It also allows players to more readily pick and choose the missions they go on, and brings the missions they consider fun to the forefront. It also streamlines other things like turning choice enclaves hostile, if that's what you're aiming for. Lastly, it opens up possibilities to sweet talk enclaves down so they're not pissed just because they just had to have your ammo at a time when you couldn't really afford it. (not that they really needed it considering you can trade it right back from them, but hey, that's Hollywood.)