Because the reason child pornography is illegal is not that viewing child pornography is bad, it's that producing it is clearly wrong. The producers are hard to hit directly, but one can reduce their revenue by making it hard to consume their product.

Producing virtual child pornography is not obviously wrong. But no politician will speak up, because THINK OF THE CHILDREN (what children? DOESNT MATTER THINK OF THEM).

Thus orwellian thoughtcrime.

EDIT: I'm willing to agree that leaving real producers an escape path by letting them claim everything was virtual is bad. So the US clause for 'realistic' virtual porn is okay in my book.

But in the UK stylised drawings of child pornography are illegal. That's just silly.