I'm not sure if by "why" you mean "what's the reason behind it" or "what's the source for this custom". If it's the latter, the Rema to Shulchan Aruch Orach Chaim 624:5 writes:
אין אומרים תחינות ולא צו"ץ מי"כ עד סוכות
We don't say Tachanun or Tzidkasecha from Yom Kippur until Sukkos
The Mishnah Berurah (§ 18) adds:
This exact type of instant noodle soup is addressed by Rabbi Yoni Levin here
Ultimately, there are two potential pitfalls that making such soup on Shabbos would present:
Bishul - cooking
Makeh B'Patish - 'The final hammer blow' i.e. making something fit for purpose.
So, he walks us through both potential melachos and how to avert any chilul shabbos.
Rav Shmuel Wosner zt"l addresses this in Shevet HaLevi, cheilek dalet, siman 23, os 5. He says that going out in the sun in such glasses is not considered an act of tzoveya.
בענין המשקפים שבאור השמש נעשות כהות אם מותר ללכת בהם בשבת - לבי נוטה יותר שמותר. ובמק"א נחזק זה אי"ה בראיות
With regards to glasses that the light of the sun makes go ...
HaRav Chaim Bleier shlita in his Chukei Chaim series here (on page 2) addresses this question.
He writes as follows:
Adding Croutons or Soup Nuts to Soup on Shabbos
Croutons and noodles. Deep-fried soup croutons, which are considered cooked1, may be added to soup on Shabbos, even if it is yad soledes bo (מאור השבת ח"ב סי' ח' סק"ה) since we pasken ‘...