According to the script, the cult's failure to kill their victim (completing the ceremony) resulted in the demon becoming attached to an object rather than erupting fully-formed into the world.

Additionally, the cult lady's soul wouldn't have been a useful conduit for the demon because it wasn't 'innocent', hence why the cult couple needed a sacrifice rather than simply sacrificing each other.

CARL: The Order of the Ram -- based on what you told me -- this ceremony they were attempting required them to take their own blood and the life of an innocent to take the soul... So that explains why she killed her parents and went after you... a pregnant woman. Nothing more innocent than the life of an unborn child... But since you're sitting here, they didn't complete the task.

MIA: Something came through...

[continued]

Carl: And they don't attach themselves to locations. They attach themselves to objects. Using them as conduits to ultimately get what they want...

MIA: The soul of an innocent...

According to this earlier script, the cult's failure to kill their victim (completing the ceremony) resulted in the demon becoming attached to an object rather than erupting fully-formed into the world.

Additionally, the cult lady's soul wouldn't have been a useful conduit for the demon because it wasn't 'innocent', hence why the cult couple needed a sacrifice rather than simply sacrificing each other.

CARL: The Order of the Ram -- based on what you told me -- this ceremony they were attempting required them to take their own blood and the life of an innocent to take the soul... So that explains why she killed her parents and went after you... a pregnant woman. Nothing more innocent than the life of an unborn child... But since you're sitting here, they didn't complete the task.

MIA: Something came through...

[continued]

Carl: And they don't attach themselves to locations. They attach themselves to objects. Using them as conduits to ultimately get what they want...

MIA: The soul of an innocent...