Who knows? But it seems the artifact was delievered... still in a powerless state. It certainly had no visible effect on the ships which towed it -- although it might have messed with minds to its fullest ability, simply keeping a low profile.
Changing the behaviour of its own escort... might have been contrary to its rules of behaviour. A handful of ships were insufficient to sustain its primary function -- but might deliver it to a location where it could be sustained. When messed with, some ships might escape, revealing its approximate level of threat (which might bring trouble).
On the assumption that the Great Machine had even the crudest artificial intellect... upon the delivery of the passive artifact, the machine would have probably noticed it (even without Draal knowing). I would imagine that upon finding something of surprising profile, the machine would alert its custodian *quicker* than an unprepared intruder could tamper with it.
(Especially an intruder who seemed generally challenged with the operation of Normalspace computers, instead preferring to crack biological minds.)
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However, if the artifact really had telepathic powers from early on... it could have used them covertly -- to much greater extent than presented.
It might have convinced its finders --
"You have found something benign and important. You are not cautious. You wish to deliver it directly to your most central base."
It might have convinced Draal. --
"You have more important business. You are forgetting that alert. You don't want to engage whatever independent computer-based intellect your machine contains."
And it might have convinced many others.
"No. You don't trust robotic guidance enough. You don't want to evacuate crew from several Minbari cruisers, and tell their computers to make a long acceleration run before... no, better forget that thought."
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After all, the artifact seemed to have a quite narrow scale of targets -- biological mind and certain types of power supply (perhaps not supply, but large enough power grid) within relatively close limits of distance.
It seemed incapable of intruding into computer systems, or having notable effect on the power supplies in single-person fighters (or to my recollection, any Minbari ships regardless of size).
Its technology was probably tuned and focused on whatever technologies Vorlons were using during their civil war (or if it got really messy, the Thirdspace-Milky Way Incident which never grew to a Thirdspace-Normalspace war).
A ship with unconventional sources of power and remote guidance or non-biological intellect might have been outright deadly against it.
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On other thoughts, perhaps "Thirdspace" simply had plot holes.
