Trump admin claims Calif. net neutrality law causes “irreparable harm” to US

The DoJ claims irreparable harm because that's the standard for granting a preliminary injunction. You don't grant preliminary injunctions for reparable harms.

If you think such irreparable harm is unlikely, then you've already made a prediction about whether the court will grant the preliminary injunction. Hold onto that thought for a couple weeks (maybe write it down) and learn something--either you were right and in reading the denial you'll be able to match your intuition with the legal logic, or you were wrong and you'll learn something from the court's injunction order about what irreparable means in a context such as this.