The question is why did the bunker busters fail? WTF would be needed to destroy these bunkers?

[jmowreader]

19. Iran did an analysis prior to building the bunkers

Step 1: Make a list of all the nations who would elect a vainglorious douchebag who might decide to bomb Iran when things are going badly for him, so as to at least temporarily get the press to ignore his many failures. This list is not long because most nations have better sense than to elect a Donald Trump clone president of anything higher than the dog pound.

Step 2: Get out your copy of "Jane's Defence Systems" and investigate all the ground-penetrating munitions these nations have, paying particular attention to the maximum depth these systems work at.

Step 3: Build the bunker twice as deep as the most-penetrating bomb they have will go, and make sure there isn't a straight shot from an exhaust port to the power plant - you don't want your bunker to meet the same fate as the Death Star.