Tzaddikim, through their total and complete dedication to HaShem, uplift their words of Torah and Tefillah straight to the heavens. If, heaven forebid, a Tzaddik learns or prays without proper devotion or to some other end, then the words fall below the Tzaddik, and the Tzaddik is helpless to uplift them.
Only the Rasha, who is below, who determines to do true Teshuvah, through his sincere Teshuvah can raise up these fallen words and redeem them. This is the meaning of the phrase where Baalei Teshuvah stand, even complete Tzaddikim cannot stand.
To me it seems there are a number of things we can explore here. It ties into many different Torahs about Tzaddikim. I'll bring two here:
The Notzer Hesed explains that Tzaddikim can reach a level where they aren't really in the world anymore, so much so, that they are almost unable to affect any change in the world at all. This would explain why the Tzaddik cannot sink to the level to which his words (chas v'shalom) may have fallen.
Rebbe Nachman of Breslov explains that Tzaddikim can throw their voices, in effect, reaching reshaim, and even non-Jews (who have a spark of a Jewish soul and are waiting to convert) at all levels and inspire them to return to HaShem. Apparently there is a level and a way that the Tzaddik actually "drops" his words to lower levels (similar to what the Noam Elimelech described above) on purpose, as a means to lead a Rasha back to HaShem so he may do Teshuvah. (Rebbe Nachman says that Tzaddikim can even throw their voices across generations or through other people to reach yet other people.)