Any chance that it's because those spells are too good? (I doubt it, but I've never done any spell analysis).
Unfortunately, the DMG doesn't give guidelines for creating spells that inflict damage over multiple turns.
If every one of the
sword's attacks hits, it inflicts 17 (3d10) damage ten times, for an average of 170 damage.
If a
prismatic spray affects ten creatures (and since it's a 60-foot cone, it could probably fit a lot more) and they all failed their saves, that spell inflicts 35 (10d6) damage ten times (average 350 damage). And since
prismatic spray is an Instant spell, there's no concentration to be dealt with. Sure, you might not actually
have ten or more creatures in that cone, but if you do, it does a heck of a lot of damage.
Crown of stars doesn't require concentration, lasts an hour, and inflicts 26 (4d12) damage per hit, for a maximum of seven attacks, and if each one hits, that's an average of 182 damage.