Uvo L'Tziyoun

Why do we say Uvo L'Tziyoun? Why do we it at Mincho on Shabbos and Youm Touv, and on Moutzoe Shabbos? Is it because the government authorities forbade the recitiation of Kedusho and so when the government spies left the synagogue, they could say Kodosh, Boruch etc.?

Rallis

The Rokeach (Siman 76) writes that the reason for saying Uva L'tziyon is to show our yearning for the Geulah. The reason why Kedushah is there is because the Evil Kingdom decreed that we should not learn Torah and its Aramaic translation.
The Rokeach also says that saying Kedusha of Uva L'tziyon saves one from the din of Gehinnom. Therefore, we say it every day. We would say it at Shacharis of Shabbos (as we do on Rosh Chodesh) but because of the multitude of Tefillos, it is shunted off to Mincha. The reason we say it after Shabbos is, says Rav Amram Gaon, to delay the return of the sinners of Israel to Gehinnom.

MPerlman