The most common reason: those curators are on a break. Vacation, holiday, hiatus, you name it. When a curator goes inactive, new submissions to them queue up instead of landing in their inbox, and they'll receive the batch when they're back.
So your submission isn't lost or stuck, it's waiting on their return. The 72-hour response clock doesn't start until the submission actually lands, so a long wait here doesn't mean you've missed your window.
You can wait, or you can cancel
If you'd rather not wait it out, you can cancel the submission and get the credits back. Open the song from your homepage, find the submission in the list, and use the three-dot menu on that submission row to choose "Cancel submission." Important: this cancels just that submission, not the song. The song stays in place; only that one submission goes away and the credits return.