Cream was more available than butter. I guess most people would rather drive to Sweden than whip cream.
However, cream was also in high demand for Christmas, so I suspect that it couldn't offset much of the demand for butter anyway.
Interesting question, don't know why this was downvoted. Making butter is really easy using a stirrer.
Since it was a milk shortage, I think it's reasonable to assume that cream was also rarer, though I suppose it might not have been subject to the same import restrictions.
Not as I recall, we made our own butter during the "crisis". At least kt was possible to get (full?) cream