It's easy to find that e.g. the EEA does not include the Common Fisheries Policy because Iceland and Norway didn't want to have their fish stocks shared/managed in common with the EU. (Aside: seemingly as a result of these exclusions, Norway has to export most of their fish raw to Poland where it is processed, since imports of processed fish are taxed higher by the EU.)
But why is the customs union (EUCU) excluded form the EEA? Which countries oppose[d] a customs union extending to the EEA and why?
(I could find out that Norway taxes alcohol more heavily and imposes quotas on imports from the EU even for personal use, but that seems like a marginal issue. Norway conducts some 10% of its trade under FTAs agreed outside the EEA, which Norway can do because it's not bound by a common trade policy with EU; but I'm also not sure this was a substantial reason for EEA not having a customs union.)
I suppose a generic answer may be the (maximal) avoidance of political integration, which would indeed also cover foreign policy in general (also excluded from the EEA) and a common foreign trade policy in particular:
During the negotiations on the EEA Agreement, the search for an institutional framework proved to be one of the greatest challenges. The EFTA states tried to avoid any political integration that would pool national sovereignty in common institutions.
But can anything more specific than this be said about why the EEA not having a customs union?