Poor planning and the nature of jump series in general.
WSJ is based on ranks and statistics, so when a series takes off and consistently does well it's stretched to the absolute limit in order to commercialize as much as possible.
So even if Hori planned out the story beginning to end, he's probably urged to take as long as possible to get there so long as the series sells well.
That and he's had 4 different editors who all have different takes on the series, which probably resulted in a lot of rewrites.
It took him six years to even get this far, I can understand getting discouraged at the thought of spending another decade wrapping things up.
The original villain was always AfO, so it makes sense to finish the series once he's defeated.