I thought Doug was the most funny character in the series. How come he was not along with Kevin and Paul for high school years?

That's a question only the writers or Doug's portrayer Brandon Crane can answer.

Either Brandon was unavailable due to other acting roles. I know he was in the original Stephen King "IT" movie. Or the writers simply decided to stop writing storylines for Doug during Kevin's High School years.

Yeah he definitely would made a huge difference if he was in seasons 5 and 6 he would been 16 or 17 had Doug stayed we would never got to see chuck who was supposed to be like Doug clueless character