One of the reasons that Belepheobe tried to escape to another world with Finn.

Biologically the royalty can only have one child, This is the sort of check and balance system that was put in place by the world and systems they live in.

As long as the magic system recognizes that there is two ‘active’ royalty then Royal’s bodies are essentially locked up. And their reproductive abilities are stunted.

Since Finley was born as a fairy, The system registered him as an active fairy his mother couldn't just have another child. So she was pressured to kill him.

She was resistant to this idea of course and she couldn't just give him to his dad to take care of him because of what was mentioned in the novel.

There was still the possibility of him awakening his Goblin lineage so the fairies could not risk giving the goblins (who had fought many wars against the Fairies by then) a goblin that could use magic.

She thought to put him in the human world and raise him there. Which would have probably worked if she didn't die in the process.

But back to the Heloise.

Creating a new royal takes a lot of energy and magic as well as time.

Esmeralda mentioned in the last chapter (or the one before I can't be bothered to check) That the gestation period for royalty takes a lot longer than normal. And it only gets longer depending on how powerful the fairy will be.

For example, Heloise spent 70 yrs carrying Belphoebe.

(the second longest gestation period in history)

And all that while you are consciously diverting several tonnes of consciously purified magic towards the child.

The magic that is consciously processed tends to be cleaner and easier to use than that that is done passively. the royal child is essentially just a being of Pure magic. If even a bit of impureness gets to them the pregnancy essentially fails.

So for Heloise who has already been at ‘Rest’ for a few centuries. Even just awakening to crown the new Queen will be a lot of energy that she may not be able to support until more than at least half a century has passed.

If they waited for her to recover to her peak the world would have ended by then and only the Royals would still be alive,

There is also the fact that she may not want too and doesn't need too because there is already an ‘ active royal’

But to Answer the Question that Sparked these chapters.

A surrogate cannot be used simply because,

1) they don't have the magic capacity to nurture a royal child. and;

2) They're too impure. The fetus would die before the second day.