It's not - IE isn't near finished yet, so it doesn't yet support the web-standards the game relies upon... that's not the choice of the game-designer...
That is because the web standards aren't finalised yet
Chrome/firefox & Opera are all utilising web features that are not set in sttone & change from week to week. That is why FF & chrome have monthly, if not weekly updates to a new version with new functiosn built in. And is alos why both of those browsers have so many crashes and unwanted effects
Yes it is great to have extra web features but they need to be standardised & settle down a bit.
It isn't just IE that won't play the game, it is older versions of FF before about 10 that won't play it and many companies won't allow the most recent versions of any browser becasue they need time to check & make sure that current applications will still work in it.
As soon as you introduce new features & standards & deprecate or remove old features, then existing websites fail to work or display correctly & even worse "in house" programs that rely on a browser won't work & some of those cost millions of pounds & can't be easily upgraded