Originally posted by Thomas Riker:

By...whom? They're basic forage. And there is absolutely no way the old Norse weren't eating them. I'm sure everybody everywhere did up until widespread agriculture.

The discovery of South America and the potato pretty much made consuming cattail tubers obsolete. Potatoes grow in poorer soil, require less water, do better in both warmer and colder climates, produce far more tubers per plant, and do not degrade as quickly when stored.

Once a culture relies on agriculture instead of gathering, use of things like cattail tubers and acorn flour decrease since barley and wheat are more abundant, store more readily, and are easier to turn into flour. Since the Viking age was in the late iron age less flexible foods like cattail tubers would only have been gathered by poorer villages. The rich coastal settlements would have relied primarily on agriculture, fishing and animal husbandry and would have turned their noses up at cat tails as a poor person's food particularly since trying to use them to make alcohol is a tedious and messy project that requires the addition of barley for success. It would have been far easier to use the more abundant barley for the entire process.

Edit: Cattail pollen would still have been gathered for use as a spice and the tubers would have been left alone for the next year's spice crop.