Because, no disrespect intended, the Blue Square divisions are a lot stronger than the Highlsand and Central District League. That, and you can't be promoted from them, which may cause issues.
Lower divisions than the Blue Square are very, very difficult to scout and to code. The players are amateur, making them very poor. With the current 1-20 system would there be any differentiation between player qulaity? That, and the massive turnover of players at that level make it very, very difficult to accurately model.
Plus, if you model the Isthmian etc., you then need to scout and have enough players at the level BELOW that in order to ensure promotion/relegation works properly. That's an even bigger nightmare than doing the 7th tier in the first place.
It would be nice to do. But I think it would be very, very difficult to do and require an awful lot of scouting.
Which is the other reason why I think the Highland League will not be forthcoming in the near future.