Having worked at a bank I can tell you, most of it is just forgotten money, or the owner of the account just isn't paying attention to the fact that little to no interest is being paid.

Years ago (as recently as around 2008) these were still viable products and a benefit to the client when the bank moved money out of a regular bank savings account at say 2% to a money market fund yielding 3%. Only caveat was a loss of CDIC coverage, but people were desperate for that extra 1% or so. Then they just left that old money market account kicking around and never did anything with it. That's where they are now.