Ineptitude
We already have a Diverted Profit Tax that is higher than Corporation Tax and is designed to tackle avoidance by big corporations. Problem is that this tax is rather discretionary, so HMRC would need a nudge to go after Amazon. Given the family connections of current Chancellor to Amazon and Jeff Bezos, this is going to be unlikely in my opinion.
On a side note - it is funny and tragic how Labour can think that even more tax burden is going to help small business. Big corporations have an army of top class accountants and solicitors and probably have people who created the tax rules on call as well, to help them avoid paying. Small business can't afford such help.