Part of it is playstyle - When I'm healing through those big pulls the tank is only going to use their "take me to 1HP" ability when I have no instant casts available, my Thin Air and Presence of Mind are down, and the 2.5s cast time and 1k MP cost is not going to be enough to get them back up before they go die and we wipe. I've had this happen often enough that it's basically become habit to keep that one big "oh sh!t" button back and simply maintain the party using my shorter cd abilities. I could get into my on personal philosophy of the best way to heal (for example: I don't like dumping resources when I have other answers available), but that would make this more of an essay.
Part of it is potency effectiveness - Sure, I can spam them with Cure II, but again, personal experience has taught me that sometimes even C2 hits like a wet noodle. I haven't been able to figure out a pattern for when or why this happens, but I have had times where I can literally spam C2 on a tank and their health bar. Will. Not. Move. It's not lag. It's not me disconnecting. It's simply that, for whatever reason, 20k healing = 0 healing.
Part of it is timing - This rolls in with the other two parts. A single cast of Cure II, when I cast it, has a base healing amount of about 20k. If a DRK uses Living Dead and drops to 1HP, I then have 10s to heal them the full amount of their health, let's call that 100k. If C2 takes 2.5s to cast, and does not crit, I will fall 20-40% short on healing, and they'll die from the effect, and burn through 50% of my mana doing so. The *only* way C2 spam works in that situation, is if I manage to precast both Thin Air and Presence of Mind. If I fail to do so (and being honest, that is the more probable scenario), then the tank is dead and I have to use a further 25% of my MP to get them back up, then use whatever I have left to keep the dps alive while the res animation finishes and the tank gets in to grab aggro back. Now, I don't know about other people, but personally? That's not how I like to do things. It's a huge waste of mana, and has a very high probability of wiping the party. On trash.
For Super it has to do with playstyle again. Tank drops below acceptable threshold = panic heal.