I don't think that parents don't care about language arts. The issues in language arts are different than math. For myself, I don't really care about my DD earning high school credits in middle school. My concern is that my DD is learning and being challenged.
She is gifted in both language arts and math, but she is only subject accelerated in math, and I don't feel the need to advocate for subject acceleration in language arts for many reasons. I find that in my DD's school, language arts classes are much more differentiated than math classes. My daughter chooses what she reads in language arts, for example. She is not prevented from reading more challenging texts nor from writing in a more advanced writing style and in more depth and complexity, even if her classmates can't do the same. I do realize not all schools are like this. However, I think parents find it easier to offer challenging material at home if students are being held back to the level of the rest of the class. Many parents have math anxiety. They may find it easy to go to the library and pick up higher-level reading material and to discuss that material with their child. However, they may be too intimidated to try handling AP Calculus or linear algebra at home.
Many parents won't see the need to advocate for receiving high school credits in middle school language arts because that won't hold them back later from receiving college credits in language arts in high school. Students need to start high school math in middle school to be prepared for college math in high school. However, in English students only have to read and write well enough meet entrance requirements and being able to do so is far less dependent on taking a specific language arts class.
There is also the concern of the content of reading material. Although a fifth grader may be capable of reading at a twelfth grade-level, that doesn't mean the child is mature enough to deal with some some of the adult subject matter adult books contain. I still keep an eye on what my DD reads. I don't want her reading sexually explicit material or graphic depictions of rape; she simply too young. I think this is where you'll find the biggest roadblocks to offering high school language arts classes in middle school. I think you'll find many parents and school personnel object because the subject matter is to mature for young students.
