Because there is something wrong with the workflow. Unfortunately you did not give us any details so we can not know what you are doing wrong. It may have something to do with adjustment layers and collapsed transformation.
Ok, what other information do you need? A precomposed comp is placed into a new comp and it shows dark. When there is text applied in that precomposed comp, it shows fine but still the rest goes dark.
If cycling collapse transformations doesn't fix the problem we will have to see the timeline with all properties of the layers that are giving you trouble revealed. Just select the layers and press the U key twice. We need to se what is in the nested comp (pre-comp) and make sure that we can see blend modes and the collapse transformations columns.
We also need system, os and AE version details down to the last decimal point.