Why not use the best of the DLC materials?

Coated items would last too long and so destroy the established market for repair and replacement ?

They are a bit more expensive to apply?

The best materials are protected by patents for which licensing is not certain?

The shaded area of the DLC composition of diamond coatings is protected in the US, in the EU, and in Japan by Patents US5,411,797, US5,478,650, EP0527915 and JP3203249 (Serial No. 509851/91) no matter what is the process for making the material.

The licensing of a valid patent protecting a good DLC material adds value to the coating.

Such a license protects the manufacturer as well as the user of the coated item from subsequent infringement litigation.
It distinguishes the coating from the "public domain" materials by certifying that the producer is legally able to make and sell the top quality formulations of DLC and ta-C.

Many patents have been issued for production and use of DLC materials, but to truly offer protection to the producer and to the end-user the patent must have been shown to be valid, by surviving in a vigorous infringement case in the courts.

Patents US5,411,797, US5,478,650, EP0527915 and JP3203249 that protect all hard carbon coatings whose compositions lie in the shaded areas of the figure to the left were tested in the case 2:04cv38 Carl B. Collins and Farzin Davanloo vs The Gillette Company filed in the US District Court for the Eastern District of Texas, Marshall Division in 2004 and settled in 2005 by mediation satisfactory to all parties.
Those patents have been proven valid and are available for licensing at normal rates.

Public document US Securities and Exchange 10Q Form filed by Seagate disclosing alleged patent infringement of all hard disk drives for using protective coatings of DLC. The asserted patent US 6,805,891 claims limit use of ta-C for protective coatings of recording media.

A different situation arises here.

Seagate reports in a public document that a lawsuit, case 2:07cv25 has been filed against all of the manufacturers of hard disk drives alleging infringement of US 6,805,891.
That patent appears to describe the application of protective layers of top quality DLC material onto media storage disks.
This patent has not yet been proven valid by surviving previous litigation.
License availability is moot because all known potential users have already been named in the lawsuit.

The answer to "Why not use the best of the DLC materials?"

There is no reason if you look for the robust patents that can:

Distinguish the coating from the weak "public domain" materials by certifying that the producer is legally able to make and sell the top quality formulations of DLC and ta-C.
Protect the manufacturer and the customer from the difficulties, so well stated by Seagate, that arise from infringement allegations.

Of course, the owners of those robust patents must offer them for licensing at reasonable rates so that the best compositions of DLC can enter the marketplace to reduce the economical and ecological burdens of repair and replacement of coated articles.