... or it could simply be an overlook by Larian.
True, this option is allways present.

If we can get the artifact, why not some random mindflayer?
No idea ...
Why can we easily walk under the sun, but its unpleasant for Mind Flayers?

(Rhetoric question.)
It may be bcs it radiates some energy that harms them ... it may be bcs its defending against their touch, but not ours, since we protect it (even unwillingly) ... it may be thousand more reasons, or none of them at all.

And yes, they know about the artifact - as was stated a few times, US warns us of opening SHs pod, she is sealed in by a console and the gith attacked and followed the mindflayer ship to Avernus (which is strange as Lae'zel admitted) - so it can be assumed that it was known, that the artifact was there (with the gith not knowing, where on the ship).
Im sory i dont follow ...
Where it was stated? O_o
I also fail to see any conection to the rest ...
- Us warns us that she is supposed to be locked in ... that is certainly true, but there dont need to be any connection to the artefact, if someone is in jail, people who put him there usualy dont want them to be just let out.

- Shadowheart is sealed ... that is also true, but once again we dont *know* why, we can certainly speculate it have something to do with that box tho, but that is just our speculation.
- Gith attacked the ship itself ... and acording to Kithrak, they have no idea about any woman who should keep the weapon, all they knew was that it was on that ship.
And I'm pretty sure, the mindflayer would have had enough time, to take the artifact from SH, since they had time, to infect all those people in teh ship with tadpoles after teh raid from teh prologue.
Agreed ...
So since time was not the essence, there was probably different reason they didnt take it.

If we put aside that option that Larian simply made a misstake ...
I think its quite reasonable to presume they cant simply take it for "some reason" whatever that would be ... so they decided to lock up Shadowheart to especialy guarded pod (we can call it jail for its purpose) and leave her be, until we reach Moonrise Towers, where Absolute makes her True Soul.

And once she will ... they will posses the weapon.
