I'm not 100% sure of the timeframe, nor of all the reasons behind it, but funding did have a part.
First, some of the "regular" teachers were assigned reading workshops. I was one, and to be honest, I was totally unprepared. I had never had a course in my life to teach me how to teach reading. In the end, I had to protest. It did cause quite a stir, and by the time the issue was legally settled, I had run the workshop all year anyhow, trying my best to help the students assigned to me. I never had to do it again, and I really don't feel too happy about it all, but I tried.
It was one of those things that happen when test scores become so important that a school is driven to all kinds of inventive ways to assure that enough students pass. The last thing we wanted was to be labeled as a "school in need of improvement" and end up on the State's "hit list" for school takeovers.
As a Vocational/Technical school we were at a decided academic disadvantage. Our students were not particularly scholars, at least not in relation to schools with high populations of college bound kids. We worked hard to help them pass the tests and had remarkable success in the long run.
Somewhere along the way, the concept of team teaching emerged. We "regular" English teachers were assigned one of the remedial teachers as a classroom partner. Mine was a wonderful teacher and she and I clicked immediately. We managed to find all kinds of ways to work together in the classroom both sharing teaching duties and taking turns.
On the other hand, I have to admit, no one really spent any time teaching us how to team teach. We were just kind of thrown together in the classroom and left to figure it out on our own. Some teams did just fine and others did not fare so well. Since then that process evolved into classes where special needs students were added to regular classes with a certified special needs teacher team teaching with a teacher certified in the particular subject area, but not special needs. I never experienced that, but again, the program had mixed results.
Such teaching methods really do require a great deal of extra planning and time to work well. Usually, our teaching schedules afforded none.
No Child Left Behind inspired the next phase of the high stakes testing experience. I didn't improve things one bit.