But as we saw in ESB, asteroids can do severe damage to a shielded SD.
After the ISDs were in the asteroid field over an unknown amount of time taking an unknown amount of impacts. And we only see the asteroid exploding and the transmission beeing cut off. The asteroid could have simply damaged only the communications or the shield interaction could have caused interferences.
Watch empire again. The hologram of the officer on that Star Destroyer puts his hands up in a posture of defense from something out of view...right before the hologram disappears from Vader's display. It looks like a lot more than just communications was damaged.
Edited by HawkZ71