Because curing it and restoring the Dreaming City to its former glory would take development resources Resources they don't want to spend, so they thought up a half assed lore reason like "Savathun kind of forgot how to lift the curse"

For gameplay reasons, it is a major expansion location (and a survivor of Luke Smith's Sunsetting), and the loop changes the location each week with new secrets, Ascendant Challenges, and enemies in the Blind Well. Breaking the loop would either mean retiring the location, or creating a new fourth instance as either a public space or new version of the location. Story-wise, there is no known means of breaking the loop. The Curse was woven by a wish granted by a Taken Ahamkara, and it also involved the time-looping powers of a Taken Vex Mind, Quria. Even with both gone, it's woven into the fabric of reality around the Dreaming City. Canonically, the curse is tied to Dul Incaru, and killing her the first time was the trigger for the curse to "reset" the time loop. If we don't kill Dul Incaru, there is a risk of her finding what she is looking for, and that is a risk we haven't taken for the past six years and more. If she were to find a way into the Distributary, or out of the main universe into whatever reality is above it, we don't know what either would entail.

If cured from the dream-state curse, the city would then be a woke city…and Big Sister will dye her hair a sparkly purple shade with blonde highlights. At which, the three-eyed moon witch will clearly see what’s what… And Shax will get another horn. Speaking of horns…we want them back! 👈 Ease Spwings! 😉👻

The loop can't be broke it was done by the wish of a wish dragon even riven said it can't be undone even by the power that created it. Basically a wish from the wish dragons are like a fundermental law of the universe once written they are made forever. You would need a horde of powerful wish dragons just to maybe break it and the only wish dragons left are eggs or hatchling non have that level of power even working together. Savathun created the perfect trap in the dreaming city and by doing so trapped her daughter in a cycle of death.

Why does Cayde-6 still voice com in strikes? Why is the Leviathan still over the Moon? Why are there Frogs on Nessus? These questions and many more will be answered ..... One day .... Probably ... Maybe .. or [spoiler]not[/spoiler]