I am a Doyle grad and now live in Atlanta, but will try to answer for the lack of community support.
First, there was the merger. I attended all 42 football games (one playoff and one bowl game) and some basketball games during my time at the school. After I graduated and before the merger, I attended several Doyle games but since the merger, only one. You see, my high school no longer exists. No more scarlet and gold, no more Pioneers. On top of that, it was merged with South Young, perhaps our biggest, most hated rival. Imagine if Duke and North Carolina combined to form a school called the North Duke Tar Devils. Would the Duke and North Carolina fans support this new school that is half what they used to love and half what they used to hate? My answer is no.
Second is geography. The school is located far out in the county away from the majority of the school's students. I grew up closer to UT than to the high school. It is much more convenient to cross the river and go to a UT game or hundreds of other options available in Knoxville than trek out to the school where there is little else. This has improved over the years and will continue as Knoxville grows and development moves further out. When I was at Doyle, we were surrounded by cow pastures and could frequently smell manure. At least today, there are neighborhoods there.
Third is success. Neither Doyle or South Young were powerhouses in anything. Yes, there was an occasional championship here and there, but that was the exception rather than the rule. The merger brought hope of being able to rival the other big schools in the area, Farragut, Bearden, etc, but it did not happen. An average program plus an average program equals an average program.
Success cures everything. Develop a winning tradition and the geography won't matter and the old wounds from the merger will fade.