The Stuart seems expensive for what you get. The AI isn't great, especially against units in cover. The sight is only 35, not like the Puma's 50. Sure, the AT is good enough for a Luchs or flamer HT, but that still isn't a compelling reason to buy it. More people would use it if it was "good" at something, but it isn't.
Here are the notes from the last major change to the Stuart:
Stuart
We felt that the Stuart was over-performing in its role as an anti-infantry unit and as a strong light vehicle counter. Furthermore, its stun rounds allowed it to hunt down light vehicles that were designed to counter it. Thus, we have made the following changes to solidify its role as primarily an anti-light vehicle unit.
Manpower cost increased from 240 to 270
Main gun damage vs infantry set to -25%
Main gun penetration increased from 65/50/45 to 75/60/55
MG Accuracy from 0.6/0.45/0.25 to 0.4/0.3/0.16667 (Hull & Coaxial)
MG Incremental accuracy from 1.04 to 1 (Hull & Coaxial).
Stun rounds only disables weapons and sight. Does not affect movement.
Veterancy requirements reduced from 1170/2340/4680 to 870/1740/3480
Instead of giving it just one or two nerfs, they gave it five. Changing the vet requirements is a joke when they nerfed everthing else about it.
Back then Stuart and AEC were anti-everything so both needed a big nerf to make the game less about LV wars. Also back then all the units got basically overnerfed. Like the Dshk as a great example. They seem to have learned from that though. The recent nerfs were less drastic.