^ partly because the weather isn't more reliable in July-August, Late Aug into early Sept is more stable (or was in the past). Common to get afternoon thundercloud build up in July. August is holiday time, busier all round. The trails and roads are way quieter in Sept, less pressure on refuge / B+B spaces. Partly just because I like that time of year and it makes kit selection more interesting, it challenges the bivi types.
And it's a free event - why make it more popular by choosing a time that's in the main holiday season, the numbers the event can have w/o being unsustainable or creating problems I'm not prepared to deal with is fewer than the demand.
Slightly shorter days are no problem - most riders aren't putting in 12hr+ days on this sort of terrain, they're in a restaurant or bar most evenings.