Well, as I understand it the issue with OpenGL was that it was very high-level without an option to drop to a lower level on demand since it was implemented in a complex GPU driver. This meant that drivers mostly had to guess what the intention was, and be optimized for e.g. specific games inside of the driver. With Vukan and a high-level library, you would have the option of dropping down to a lower level if you want and making those optimisations yourself. Keeping the complexity inside of the library, not the driver means you can modify it as you wish.