I mean the drainage is completely different from York Street, which it is.
As for the rainwater, I would imagine you'd need to filter it once it's leeched through the pitch and picked up soil and sand particles, not to mention whatever chemicals are already present, and anything coming off the roof will have picked up dirt/debris and bird crap and such like. I don't know, but it might be that the cost and hassle of filtering that water just isn't worth it.
EDIT - I Googled it. It's called rainwater harvesting and filtration is indeed the big capital outlay. If you look you'll find a fair few examples of stadiums harvesting water off their roofs - but we're talking Major League Baseball, domed stadiums in Japan, and Allianz Arena type arenas. Harvesting water that's drained through a pitch seems less of a thing, so I assume once it's fallen on the grass it's considered unusable.