People have no patience. They want things now.
As Durante said, other standards needed years of meetings, specifications, unification, etc.
Right now, Mantle isn't open, true.
First, I would say they wanted to get out what we have right now, a proof of concept done with a few developers to raiser public awareness and make a point of how a low level API is something good on pc.
Later this year, once they have a body of evidence of the improvements and some public supporting them, they will post specifications and initiate talks with other bodies of the industry. Then maybe in two years we will have a Open Mantle standard.
They are being smart, I think this way they have improved their chances of success. First start it privately to make a point and show your willingness to the project, and then initiate the talks. The other way, the companies could have been going on in talks that wouldn't go anywhere and ambiguous statements about maybe yes, maybe no.