My guess is they are hooked on the money they are saving. My guess is in the last few years
assemblies are costing more than they take in.
As noted above, only special conventions are cancelled through 2022, No announcement has been made on "normal" regional conventions.
Conventions are by far the biggest cash cow the WTS has. They make many many MANY millions off the contributions of grateful JWs. There is virtually no expense other than the small "off season" rental of the facility (typically a basketball arena that sits empty for most of the summer) so the profit margin probably exceeds 90%.
And the international "special" conventions are an even BIGGER gobs-of-money-maker. In addition to the contributions from attending JWs, they almost certainly get kickbacks from JW-friendly or even JW-owned travel agencies charging exorbitant rates for JW travelers' airfare, hotels, and transportation.
They will open up regional conventions as soon as possible. They must have absolutely hated to lose the special conventions for 3 years in a row (2020 - 2022).