Because she is within the huge majority in the bell curve: 80 to 120 IQ. That is perfectly adequate for her to function in society. The real danger is that the expectations of the adoptive parents can be too high and leave the child with enduring feeling of inferiority. It’s a blessing that you discovered this now and can adjust your teaching within the family to her level.
I wouldn’t worry about technical school or whatever. Those worries are minor. The big one is to not expect her to perform above her ability. It’s more of a day-to-day, nurturing effort rather than any particular course of action, like private school or homeschooling, although both of those could be good.
I’ve come to the realization that we all have different qualities and challenges. My older daughter with the high grades is much less happy go lucky and probably couldn’t deal with the challenges younger daughter does. Older daughter was furious when we were ecstatic that younger daughter had a report card that showed she had worked hard. Older daughter was mad that she wasn’t given more credit for her grades being so much higher. I think she’s finally understanding younger daughter works just as hard or harder to achieve what she does.