That is by design, Dirge of Ceresa is Sovereign only, so it isn't passed to familiars.
Oh wow, that's new. I remember when my champions could cast Dirge.. I didn't even notice the change since it hasn't been worth casting for several patches now. If it's going to be so limited, could we maybe buff it so it's worth casting?
Consider..
Dirge: 64* mana, 2 turn cast time, 3 + 1 per shard poison damage a turn for 10 turns. Only available to a sovereign with a specific trait.
Contagion: 18 mana, 1 turn cast time, 2 + 1 per shard poison damage a turn for 10 turns. Available with death 2 + air 2
Horrific Wail: 24 mana, 2 turn cast time, 3 damage per level of caster instantly. Available with death 2 + water 2
Now, when you add up the 10 turn total, Dirge and Contagion sound devastating - but consider that it would take 12 turns to get all of Dirge's damage in. Battles are fast by design, they're generally over by ~5 turns, either they've killed you or Dirge has killed them by the time Dirge gets 3 ticks in. And even if you mentally limit the spell to 3 ticks of damage, that damage is delayed and therefore inherently weaker than the same damage done instantly - while it's ticking (1) they have time to try and kill you and (2) you can't cast another Dirge at them. Also, it's poison damage.. many things (elementals, demons, sovereigns with 'hardy', etc) are immune to it.
So considering the delayed damage over time, not to mention poison immunities, Wail is simply better. My last game, by the time she hit level 15 Ceresa had exactly 7 death shards (corrupting every shard I could get my hands on). Her Wail does 45 base damage multiplied by various percent bonuses, her Dirge does 10 per tick times same percent bonuses. It takes 5 ticks for Dirge to match Wail.. again, by the time you've spent 2 turns casting, you have perhaps 3 ticks of Dirge damage before the battle is over one way or another. By the time Dirge has done 40 damage, I could have cast a second Wail for a total of 90 damage, all for less mana than a single Dirge! Not to mention Wail's damage seems to affect everything, or at least more things than Dirge. Note this spell is only 1 level up away for Ceresa, once you get it you'll take Dirge off your skill bar permanently.
But let's assume Wail gets nerfed, or you have an insane number of death shards with a low level sovereign, or for whatever reason you really wanted slow poison damage over time, Dirge still isn't worth casting - Contagion does almost exactly the same thing except faster and far cheaper. Granted it's a little tricky to get air magic on Ceresa, but then other champions can cast it too, all in all it's more commonly available than Dirge.
Recommendation: increase Wail's mana cost and possibly reduce damage (or at least reduce scaling.. I suspect higher base damage but less per level would work nicely). It's become the new Dirge, like Dirge was in the old days when it instantly oneshot armies, except far cheaper to cast and easier to get. Buff Dirge a bit so it's at least better than Contagion; right now Dirge (1) costs too much mana, (2) does too little damage too slowly, and (3) too many creatures are immune to it. It really needs two of those three problems fixed, it'd be useable with one weakness, but not 2-3. Contagion is a decent spell since it's better in (1) and (2) - far less mana and casts faster - and it's still not worthwhile next to Wail.
*Note I'm leaving out the benefits of traits to damage/mana cost, since they should apply to all the spells equally.
