1. Because you couldn't remote control an airship well even when you were right next to it because of it's movement system. Heck, even when you were on the airship, you couldn't really fly it that well. They can move fast, but you pay for that with the inaccurate controls and difficulty aligning to the world to rejoin it and get everything working again.
2. When it was moving, basically anything non vanilla in it didn't work, meaning no chunkloaders, and you couldn't mine and fly at the same time.
3. IC2 Miners work pretty fast. By the time you got set up, you'd have to come back 5 minutes later to move the rig.
4. Airships isn't the most stable of mods, so anything bigger than a 2x2 miner set up tended to go horribly wrong. My attempts at 3x3 set ups often ended up with corrupted worlds.