Because it's the best rugby stadium in the world and its inclusion was a cornerstone of the English bid. The WRU agreed to its use provided the English organisers agreed to Wales playing all their pool games in Cardiff - this would have brought big revenue To benefit everyone. Then England,Wales and Australia got drawn in the same pool and suddenly Wales weren't allowed to play their major pool games in Cardiff any more. It revealed a lack of integrity, cowardice and the familiar arrogance on the part of the English organisers.
Hopefully karma will prevail.
Ok, that's interesting. Weren't there potentially many of the large English football grounds that could have been used. I guess it's understandable that England play Wales at Twickenham, but the Aussies should be no problem for Wales even at Twickenham, shirley. 
https://en.wikipedia.org/wiki/2015_Rugby_World_Cup#Pool_A
The whole tournament [and therefore the grounds used and the schedule] was agreed by Rugby World Cup - not by the RFU and England. There is a lot of disquiet amongst the RFU and other rugby people about the number of football grounds being used rather that rugby grounds, notably Leicester's Walker Stadium [or whatever it's called nowadays] instead of Welford Road.
It's true that Wales supported the English bid after they withdrew their own bid. England were granted the tournament in 2009 but the venues were not finally agreed until 2013 at the same time as the schedule was announced. The WRU could have pulled out well before that.
Tonight England are playing in red and using the away changing rooms at Twickenham when they play against Fiji. They are officially the "away" team.
I don't think the RFU would have organised that if they'd had the choice!
