My best guess, based on reports from purported insiders, combined with common sense, is as follows.
(1) Disney has plans to switch to a system in which some or all fastpasses will cost extra money in some way.
(2) Either that system has been chosen, but there are continuing IT problems getting it fully realized and integrated with their current systems, or Disney is still debating internally how that system will be structured
(3) Disney could more or less snap its fingers and reinstate the old fastpass+ system right now to improve the summer guest experience.
(4) But Disney would very much like to announce the new system as an improvement on a status quo of no fastpasses—a unique opportunity presented by the pandemic that Disney doesn’t want to waste. (Of course, Disney will in any event market the new system as an improvement over everything that has come before. But they fear the negative publicity and guest anger that could come from charging money for something that used to be free.)
(5) Even if the new system is ready to go—or will be soon—Disney seems intent on holding back everything new until the 50th, so as to make that event as special, new, and exciting as possible.
Of course I have no way of knowing whether any of this is right. But Disney has not given us any other explanation. And if this is even close to correct, I think it’s unconscionable.