Apple

It's surprising that Apple isn't listed (among those backing the scheme) given that the company designed the HomeKit standard with security and privacy as key objectives.

I think that's the conflict: Apple has its own certification programs. From Apple's perspective, a successful government-backed trademark would compete with Apple trademarks for consumer mindshare and the certification would add new overhead to Apple's own product launches.

Other brands backing this program have more to gain than lose from it, e.g. because their own certifications aren't as well marketed, or because it simplifies product screening, or sets up new hurdles for competitors. Apple's in a unique position where none of those benefits are relevant. It only sees the costs.