The Meshech Chakhma offers two answers.  Here is one of them. 

Why the poles could never leave the Aron can be understood similarly to the Rambam’s approach to lighting the menora. According to the Rambam (Hilkhot Temidin Umusafin 3:10) the candles were lit both at night and in the morning.   

There is a powerful lesson behind the Rambam’s approach.  The Gemara comments:  “Does He (G-d) need the menora’s light?  Of course not.  Rather, the Menorah’s light was testimony that the Divine Presence rests in Israel.  (Shabbat 22b)” This might be what is behind the Rambam’s understanding of the mitzvah of lighting the Menora.  Candles lit during the day are clearly not there to illuminate the darkness.  They are a sign of the Divine Presence.   

The poles of the Aron might serve a similar purpose.  The Talmud tells us the Aron’s weight was not carried by those holding onto the poles.  “The Aron lifted up those who carried it (Sota 35a).”  Its poles were not there to enable it to be carried, but for its transporters to hold onto as it was miraculously moving from place to place.  The Aron is the dwelling place of the Divine Glory, which itself carries all the worlds.  

Therefore the poles are always to remain in the rings of the Aron.  This makes a powerful statement.   Just as the poles do not serve to transport the Aron when it is stationary, so they do not when the Aron is being “held” on the shoulders of those who “carry” it.  It carries them; they do not carry it.   

Just as candles lit during the day are not just there for illumination, so poles that remain in a stationary Aron are not just there to facilitate transportation.