Rashi: It can be compared to a country that rebels against the king, As long as only a minority rebel, his entourage remains intact; but when they all (or the majority) rebel, his entourage falls apart. So it is here, as long as only the Kohen Gadol has sinned, the Kedushah of the Mikdash is not affected; but when the entire Tzibur sins, Chas ve'Shalom, it departs.

Oznayim la'Torah: Because, as opposed to the Kohen Gadol, whose main function is Avodah - irrespective of his suprior wisdom, the Sanhedrin's main function is that of Hora'ah (issuing ruling), and whose error in that field cuts deeper, sufficiently to remove the Kedushah from the Paroches. ¹