There are hundreds (thousands?) of registrars you can choose to do business with. Meanwhile, there's only one power company that's run lines to your house, only one water company running pipes, and you have a choice of 2-3 ISPs if you're lucky. Those are utilities. A random business service provider with hundreds of choices available on a competitive free market is not a utility.

A key argument for ISPs being treated as utilities (which is, itself, a controversial idea) is that they tend to be local monopolies or oligopolies. This is not the case with domain registrars, a field in which there are many available alternatives selling an interchangeable commodity product.