According to the notes in the Artscroll machzor Chayim Yechezkel,(p. 406)
The reason lies in the special nature of this Kedushah which is based on Pirkei D'Rabbi Eliezer's narrative of the angelic praises...During Mussaf of the Sabbath and of Festivals, when we recall the additional Temple offering that symbolized the higher level of holiness that we were privileged to enjoy, we are permitted to combine our own tribute with that of the angels. On Yom Kippur when, as noted on page 70, Jews are indeed reckoned like angels, we are worthy enough to recite this Kedushah during every Shemoneh Esrei.
The note on page 70, when discussing why we say Baruch Shem K'vod out loud, reads,
On Yom Kippur, however, when Israel elevates itself to the sin-free level of angels we proclaim is aloud as do the angels (Devarim Rabbah 2:36)