In the age of streaming it can just be quietly released any point just to mark a check mark of "new content this month" for that streaming service whether Netflix if contractually obligated or any other. No promotion as to not take away from the current products. Clearly this should've been released about 2 years ago when the show had finished airing its second season but before the third but since season 2 was cut short and they moved on this just sort of lingered since it was already a thing that was happening but since the media and fans don't care they've been able to just keep it on the downlo, similar to Star Wars Detours which has a completed season and Disney still hasn't released it but surely will at some point to have more content at Disney+.
As far as Power Rangers, no one knows exactly why Disney would sell an IP they owned, however they didn't fully own it as TOEI has some rights associated with the brand and they own the Sentai costumes so Disney never owned 100% Power Rangers or ever would. It's all speculation but it could be that they're obligated to do something with the brand or pay TOEI royalties for example if they kept it so they decided it wasn't worth the headache for the little money the brand would give them if they kept it to rot in the Disney vault and they now owned Marvel.
I'm not sure where you're getting your information from but power rangers was relatively successful under Disney and more successful than it was before the sale and even after the sale. Power Rangers became the #1 brand for toys for boys at least for the first few years they had it, they kinda stopped caring near the end. Saban bought Power Rangers back and there was a big push in marketing to make them relevant again but as much as Saban did it he was never able to reach the highs of the Disney years, Nickelodeon and the two year format proved to be devastating for Power Rangers. Granted
Power Rangers became cool again a few years ago but that wasn't because of anything Saban did, it was because of nostalgia. The kids who grew up with Power Rangers in the 90s were finally old enough to LOVE Mighty Morphin all over again as a manchild. None of those older fans care about Power Rangers, they care about the new Mighty Morphin Power Rangers nostalgia. And it's because of that 20 year cycle brands have to recover old fans who are now adults with jobs that the brand became big enough that Hasbro paid top dollar for it, not because of what Saban tried to do and failed at.
Power Rangers had original content every year since its inception because it adapted Sentai, now that Hasbro owns it, they seem to only be interested in the Mighty Morphin season so basically all the world building and continuity that has lasted like 30 years will be destroyed in favor of nostalgia and remaking MMPR over and over again like the Boom Comics have made.