If you read Zariel's background and current plot that WOTC presents to us, she is a very troubled Archdevil, she faces abnormal civil strife within her own ranks (more so than the normal Archdevil, which must be really bad, if that's being stressed out). That is mostly due to her background, which makes both devils below and above her distrust her (again, more so than the "healthy" distrust between devils).
Furthermore, we know Karlach's heart engine is an outdated model when speaking to the gondsmen at the factory in the Avernus part of the game, which means that Zariel has access to much better troops to send over to the material plane.
Moreover, the people she sent over to capture Karlach in the game are pathetically underequipped, Karlach doesn't really get hunted much more in the game, this implies Zariel might not really give much of a crap about Karlach at all.
Another clue to her background, we have even the plot about Raphael trying to oust her, which just ties in with her WOTC cannon.
And lastly, as it was already pointed out, Tav stands in a unique position of possessing an extremely powerful soul that Zariel wouldn't pass over, she would likely free Karlach and restore her in exchange for Tav's services and his soul :P
