On Lev. 26:19, ושברתי את גאון עזכם - "I will break the pride of your strength," Rashi (citing Sifra) explains that this refers to the Beis Hamikdash.
But this is still fairly early on in the cycle of increasingly severe sins and punishments; it's two more cycles (vv. 21-22 and 23-26) before things get to the point of the Jews being besieged, and then still another (vv. 27ff) until the cities are destroyed and the Jews exiled, which is when the Beis Hamikdash was destroyed. So why is the warning of this event placed so early, instead of somewhere around v. 30?