Probably because it's not there. There is no canon mention of it. Nobody mentioned the name or that it should have been there in the first place. The only indication of its existance is the brief glimpse of its location on the underside of the saucer section. The most likely in universe explanation is that it wasn't installed yet. We know Voyager was brand new. It's possible they were in a rush to get Tuvok and left on what they thought would be a two week mission without it.
Either it didn't work for some reason, or it was completely missing and what was visible on the bottom of the "saucer" was the outer doors to the areoshuttle's berth.
The same reason they never used the second warp core or second computer core when they would have saved the day - they didn't exist outside of the MSD graphic at the back of the bridge, and the art department's imaginings. Of all the Trek's (except maybe Discovery, where I'm still not sure where Starbase One is meant to be), Voyager seemed to have the biggest disconnect between the art department and the writers.
second warp core? So that's what htat is in the graphic? And then Concerning flight would be different because they are chasing a singular computer core (but they had two? Was it a back up or for other functions therefore two is essential?)
This was at the back of the bridge and duplicated on several stations all over the ship, and visible in every episode. It's got two warp cores and two computer cores, directly conflicting with "Concerning Flight" and "Day of Honor"
I have been a Star Trek fan as long as I can remember, seen every episode, saw Caretaker when it was first run. This is literally the first time I have heard of the Areoshuttle. Just...wow...I'm blown away.
But, I am literally still stunned that it is right there...every time you get to see a dorsal view of voyager. Crazy...never realized. Would have been cool to see in the show. Since I realized this after seeing it this morning, I watched a video on youtube of the screen tests for the areoshuttle, it was awesome. The design reminds me slightly of a maquis raider.
I thought it was part of the looks. I didn't even register as an outline of a smaller ship. Now I can't unsee it.
As many here have already mentioned, there is no canon answer as to why it wasn't used, or for that matter that it even existed at all, since no one ever mentions it.
If anyone hasn't seen it, here is the test footage. Interesting that the pilots are from DS9 I wonder why they never used it since they went through the trouble of creating it and animating the footage