In addition to what the other reply said: chicken and egg.
Professors on tenure track want publications in highly-rated conferences and journals. Professors who have tenure want their students to get publications in highly-rated conferences and journals. So when they have a good paper, they want to submit to them.
The prof or grad student isn't directly paying the costs of closed journals, so why would they take the risk of submitting their best work to a journal that at best is untested when they think they can get it into one that is highly-respected?
For people to submit good papers to it, the open-access journal needs to be well-respected; to be well-respected it needs to publish good, impactful papers; to publish good, impactful papers people need to submit said papers to it.
Probably we'll get there one day as the quality of papers submitted to open-access journals gradually increase (because while there is little incentive to submit to them, many people will consider little to not be zero), but it'll take time.