There are many possible reasons (probably covered already in this thread)
1. The insane job hours they must have to take care of their child
2. The lack of teachers really giving progress reports on the child never really caring why the child may be failing because they get paid either way
3. Lack of education themselves and not understanding what the child is studying
4. Life- Problems in their personal life that consumes them and leaves the child to his/her own devices
5. Some of them just shouldn't have had a child....
But they can be many more reasons but these are some of the things I have seen and experienced myself growing up. Things today are harder for our youth and we (society) have been trained to use school as nothing more than "daycare" you are forced to send you child to school most of which don't teach them anything but how to "superman wit' it" or what boy's have that girl's don't.
We put so much trust in schools to educate our children that we fail to take extra care of our own.