Because, if not, the question isn't between aspartame or some other interchangeable artificial sweetener, but between aspartame and HFCS or regular sweetener, which I strongly suspect contributes more deleteriously to health via obesity than aspartame does via possible carcinogenic affect.
The thing is, I'm okay with governmental bodies regulating substances (not necessarily the WHO, but that's a different story), but you need to balance different factors including substitution when you make a decision like this.
For example, the EU recently banned topical zinc as ZP in topical formulations (think head and shoulders) due to being a possible topical carcinogen (the evidence suggested it wasn't, but it is a non-topical carcinogen) because interchangeable formulations existed that also worked.
Given that it probably isn't possible to subject a substance to the same scrutiny as aspartame without many years of targeted and expensive effort, we're probably just stuck with this sweetener for the foreseeable future.