Why is a baal teshuva on such a high spiritual level that even holy tzadikkim cannot achieve it?

The answer is given by the Rav in one of his most moving teshuva lectures, given in 1969 and just posted on Ohr Publishing, the site dedicated to videos of the Rav zt"l.

Part of this teshuva drasha is summarized in Pinchas Peli's 1974 classic edition of the Rav's thoughts on repentance, "On Teshuva" - (in Hebrew: Al Hateshuva) in the last two paragraphs of the chapter on "Expunging evil or elevating it - how impurity can cause purity.".