Presumably inertia. Bureaucracies are not productive enough to enact changes like Brexit in a short amount of time. Europe will need to determine exactly the

Presumably inertia. Bureaucracies are not productive enough to enact changes like Brexit in a short amount of time. Europe will need to determine exactly the ruleset which they believe will apply to UK imports/exports, and it will be extremely costly and slow to make any significant changes, since whatever is decided will need to flow down to multiple agencies. Consider, for example, the cost of rewriting lots of technology to support a new arrangement for the UK. Can it be done in 2 years, 4 years, 6 years in every single country that the UK trades with? It will be easier to have arrangements to continue with what we have for now and then to grandfather in what already exists at some point. (This is just my layman's perspective and it's based entirely on intuition so I could be completely wrong!)