I could be wrong, but I think it was because she could be part of a "community" again.

Because she was fed up with being a good girl. I mean, look what it got her.

She was stuck out in the sticks alone with her starving, religious zealot family and being the eldest was also saddled with the responsibility of cooking, cleaning and caring for her younger siblings.

The devil offered her a way out with the allure of food, freedom, seeing the world and power.

That's the most logical and practical thing to do.

It's an allegory for coming of age and embracing one's sexuality... she has basically become a woman and has retaken ownership of her body and embraced the pleasures and risks of adulthood...

she has basically become a woman and has retaken ownership of her body

how is retaking ownership of your body an allegory for coming of age? That's kind of weird.

she realises that her body is hers to do what she wants with and no longer subject to strict parental control. she has come of age. has become an adult... a woman, not just a girl...

@wonder2wonder said:

Why did   Thomasin   sign away   her   soul?

Because she was a dumb ass. She had basic, vivid conformation that she was being courted by the devil and knew she would be damned (unless she actually was saved but she lived under the doctrine of condemnation so even if she wasn't damned, she'd still believe she was), yet she did it anyway.

@wonder2wonder said:

Why did   Thomasin   sign away   her   soul?

She signed her soul away for one reason, survival. She wasnt going to make it back to the community alive, she would be killed by the natives or the witches or the elements. Not to mention if she did make it back they would probably just blame everything on her and kill her as a witch anyway. So she basically picked life over death.

@wonder2wonder said:

Why did   Thomasin   sign away   her   soul?

hysteria/shock at what had just happened to her family

Because Satan corrupted her step by step until she was left with no other option.

Everyone she relied on for her survival was gone, including heaven after she committed murder. Going back to the colony was a long journey, and there was no guarantee they'd be sympathetic to a sinful outcast.

Satan offered her an alternative to a slow starvation, and by that point, she was too far gone to resist his temptation