Was going to point out the action economy of Call Lightning vs Fireball, but
@Dausuul beat me to it. On top of all of that, Call Lightning uses your concentration.
I'm playing a Tempest Cleric in a campaign right now, and even I hardly ever use Call Lightning, despite the option to maximize the damage a couple of times. Using a 3rd level slot, and assuming the 65% chance to fail the save, even with the optimistic assumption that I could hit two targets each round, over three rounds at level 6, I can expect to do:
Round 1: 2 * (30 * 0.65 + 15 * 0.35) = 49.5
Round 2: 2 * (30 * 0.65 + 15 * 0.35) = 49.5
Round 3: 2 * (16.5 * 0.65 + 8.25 * 0.35) = 27.2
Total: 126.2
Suppose instead I use Spirit Guardians in round 1, maximize Shatter in round 2, and then Toll the Dead in round 3. I could of course maximize Shatter in rounds 2 and 3, but I want to try to roughly balance resource cost, figuring that the extra Channel Divinity is roughly equivalent to a 2nd level spell slot. Of course if I'm using Call Lightning I don't even have the
option to expend more resources to nova. I'll make the conservative assumption that my Spirit Guardians and Shatter only catch two targets.
Round 1: 2 * (13.5 * 0.65 + 6.25 * 0.35) = 21.9
Round 2: 21.9 + 2 * (24 * 0.65 + 12 * 0.35) + 13 * 0.65 = 70
Round 3: 21.9 + 13 * 0.65 = 30.4
Total: 122.3
So, for an even resource cost, even with a conservative assumption about targets (compared to a somewhat liberal assumption in Call Lightning's case), I'm doing about the same damage using spirit guardians and one maxed Shatter that I could do with Call Lightning and two maxed bolts.
Now, under these assumptions, the Call Lightning plan looks clearly better, since (a) the damage is more front-loaded, and (b) once I have 2x Channel Divinity / SR, two channel divinity uses is cheaper than one channel divinity and one 2nd level spell slot. But if I allow for the possibility of catching three enemies in Spirit Guardians and/or Shatter, plus the
option of a bigger nova, it's rarely going to make a lot of sense to use Call Lightning even on the class best suited to use it. The main exception is if you're facing flying or highly mobile creatures, in which case Spirit Guardians is tough to leverage.