I don't believe the messon T (nor nukes) is used against ground targets
Depends on the ground target.
For absurdly high value targets, I can absolutely believe that Meson-T strikes would be called in by Forward Observers.
There's no such event told about in all Traveller 3I story
That just means that infosec and loyalty to mission/cause is high ... rather than being proof it has never happened in all of recorded history.
If anything, I would argue that meson strikes against ground targets would amount to the equivalent of "small arms fire" during The War of the Ancients ... since they would have had anti-matter, disintegrators and
who knows what else kind of technologies to weaponize against each other (resulting in the higher than normal number of asteroid belts in some sectors). Shionthy/Regina has an entire belt of anti-matter asteroids, for example.
My point being that you're not dealing with a case of "if everyone died, then who told the story?" but rather a case of "if a meson gun is fired into the forest, can anyone hear it?" and the lack of news reports about such action(s) suggests that if it does happen word about it doesn't get far.
Just because I don't hear news stories every day about what spy networks are doing on a daily basis is not evidence that there are no spy networks in operation. Absence of evidence is not evidence of absence.
Also, consider what the results are on the receiving end ... if you're a reporter (or whatever). The spherical obliteration zone will have a distinctive pattern to it, but it would be something you would need to be trained to recognize (after the fact) rather than something that is obviously recognizable at a glance by the layman. It would make craters/voids that would look suspiciously like other kinds of detonation/impact weapons so it would (I'm thinking) require training to spot the differences (that was a meson gun, not a fusion bay weapon or hypersonic kinetic kill missile, for example). My point being that for a layman unfamiliar with the distinctive specifics, there are opportunities for potential confusion and misinterpretation of evidence that could not conclusively prove that an orbital strike was made by a meson gun exclusively because there are no other possible interpretations of the surviving evidence. It isn't necessarily quite as clear cut when reporting after the fact is my point ... especially if some time has passed since the engagement took place.
Furthermore, have there been any Traveller stories told in settings where meson ortillery in support of ground forces could have been a component of the narrative story being told? I wouldn't expect such things to feature (prominently) in a spy story or a "merchant run gone bad" kind of story, since meson ortillery in support of ground forces is absolutely a military/mercenary story genre.
The ship based weapons have their value to be sure, but when you're talking tactical engagements (with turns measured in 10's of seconds), ship weapons fire with a frequency of 10's of minutes.
To be clear, I'm not talking about using a Meson-T in any kind of Danger Close kind of tactical engagement support role.
But if you've got a high value fixed surface installation that you want to ... wreck ... before the troops roll in to mop up whatever survives, then absolutely calling in ortillery fire support from the Meson-T by a Forward Observer would be the way to ... obliterate ... such an obstacle with a minimum of "fuss" for the ground troops. If you can call in "artillery" of that caliber from "over the horizon" for extremely high value targets in a way that isn't Danger Close to friendly forces, I can easily imagine that such an option would be exercised when it is useful to a ground commander.
For Danger Close engagements, there's the battlefield Meson Accelerator detailed in LBB4, p46 and p48-49 at TL=15 that weigh in at 60,000kg, requires a crew of 12, has a range of 50km and an ROF of 2 per combat round(!) for a burst radius of 50 meters per shot. Pretty sure that those would get used in surface warfare engagements in support of ground forces. The ortillery Meson-T would be called in against LARGE "make that go away" fixed ground targets by Forward Observers.
Cue Queen singing
We Will Rock You track as a morale booster for the ground troops ... that's optional ...