Maybe because I learned another language at a young age and also taught that language--because I lived in another country--I've always found the connections among ELL, ENGL, and TAL, (Teaching Additional Languages). I read and write with people from more of a broad-based "literacies" background.
When disciplinary boundaries are drawn, often it is academics marking their territory, but I also think that it gets down to contextualization. Teachers are facing particular issues when teaching certain kinds of students. Until recently, we had a segregated system within our schools (and still do in some places through practices like tracking or language grouping), inclusion models, economic shifts, continued urban and suburbanization, and immigration patterns place content area teachers facing the complex issues that teachers of English to speakers of other languages have faced when working with students in isolated classrooms; similarly, those working in schools where students identified with learning disabilities who would have traditionally been taught in separate classrooms are now being taught in classrooms with students are not labeled. Combining pedagogies to reach all students in classrooms that are often the size of small lecture hall, filled with students, is a daunting task.
Add the challenge of being the English Language Arts teacher and relying for the majority of your content being predicated on the fact that your students can read and write age-appropriately. As you can imagine, the study of what the concept of English Education entails grows exponentially.