It would have been more difficult to enslave the slave of the angel of slavery if she had used the Chain of Command divine ability to enhance the power of her slave all the time. Legion would have still succeeded in stealing her slave, but it would have been more difficult to do so.
To be fair, the slave they picked was at rank 7. So it would have taken a lot more spiritual energy to raise it to rank 9. This was why she couldn't enhance the power of the slave all the time and one of the reasons why Legion picked that specific slave.
Legion doesn't want to turn out like her, and they also want to be able to have a lot of free spiritual energy to use for other important things. This is why it is very important for them to raise the level of their slaves to the demigod rank as soon as possible.
