Could just be sustainability related again perhaps?
They're in a very rural area with a limited catchment area, so maybe he feels they just don't have the capacity to grow much beyond a regular 5000 unless they draw Man Utd in the cup or similar.
I suspect it's one of those situations where the environmental impact from the extra 3000 seats is too large, and this way he gets both the publicity for his energy company, and has a stadium that is realistically large enough for them, and won't be a Darlington style white elephant when he's had enough and sells up in a few years time.