Does anyone know why Bonaire isn't open and when it will be?

Port agreements are most likely holding things up. All '21 and early '22 itineraries are still very fluid, lots of cancellations and re-routing for the rest of the year. Change is the only constant.

It may be COVID-related but it's hard to tell.

Yes, it's open for regular flights. Sorry should have specified that it's the cruise port that's closed. There's a few small ships still scheduled for August (if cruisetimetables.com is being accurate), but others, such as Carnival dockings, have been canceled.

The last update I could find a source for was in this article from June (last paragraph):

Delegation to Miami to discuss Bonaire as a cruise destination |

It says "possibly in September," which makes me think not in September sadly.

According to this Bonaire is open

https://www.travelweekly.com/Caribbean-Travel/Caribbean-readies-for-a-wave-of-reopenings

Bonaire

Open to U.S. travelers. Fully vaccinated visitors must produce proof of a negative PCR test done within 72 hours of arrival. Unvaccinated visitors also must produce result of a negative antigen test taken within 24 hours of arrival as well as the negative PCR test. Bonaire has added the availability of rapid antigen testing at its Flamingo Airport to help travelers adhere to the current testing protocol. All travelers must complete a health questionnaire. All businesses, including hotels, car rentals, tour operators, watersports operators, taxis and restaurants, will carry a Bonaire Friendly Safety Seal certifying that they have been inspected and approved regarding health and safety standards. Updated July 27.

Following! When we booked our October cruise one of the deciding factors was it would stop in Bonaire.