Because it hasnt had an impact all summer....
I understand the final game is a big deal, I threw a party for it, but if I was deadset on seeing Diagon Alley I wouldnt have changed my plans. I gues thats just me then. The large amount of foreign tourists does make sense as well. But I heard this from management, and they were surprised as well. So the low weekend attendance isnt quite as obvious as you seem to think.