Ah, I see your point - for some reason, while projects return their custom_fields property by default, portfolios do not.
To get the custom fields and values for your portfolio, include opt_fields=custom_fields in your request. That should do the trick.
@Ross_Grambo @Andrew-asana, any idea why this inconsistency in the API? It seems to me that GET /projects/:project_gid and GET /portfolios/:portfolio_gid should behave consisntly with each other in terms of returning the custom fields collection, no?