While I agree these are sub-par as player choices, each can be a solid choice for the bad guys. Why? Because both can cause ongoing damage if the target misses their save. And ongoing damage equals failed death saves if the character is dropped. Further, immolation turns the target to ash if it causes the damage that kills the target.
I used vitriolic sphere against a mid-level party in PotA. I didn't expect much from it, but two characters blew their saves, both dropping due to initial damage. The follow-up damage caused each to take a failed death save. The remaining party members scrambled to stabilize them, giving the bad guy more time to pound them with other spells. The party ended up retreating to avoid a TPK.
This isn't an issue when PCs sling spells at NPCs, as the rules generally assume bad guys die when they reach zero hit points. But for PCs, who make death saves, any spell with ongoing damage has an extra element of danger.