TOS family Heavy Flamethrowers

I can't say for sure but at a guess the physical dimensions of the vehicle and upper structure is limited so it will fit in transport planes and on rail cars for transport so with the cages fitted with a useful distance between the cages and the rocket tubes the available space for rocket tubes is reduced.

I would also say that the potency of the rockets has improved and instead of having 30 rockets on one vehicle, having 15 rockets means needing more vehicles but because they are relatively automated and don't need to line up side by side to fire you can have your thirty rockets coming from two separate places, making the job of taking out your vehicles twice as hard.

Precision and improved warhead power might mean most of the time fewer rockets will be needed so the launch vehicles can carry fewer rockets.

With longer flight ranges they can be fired from further back which massively increases the area you would need to scan to find these vehicles before they fire.

Once they do fire having multiple vehicles means they can scatter and disappear easier too because they are firing fewer rockets so they will be empty sooner and on the move quicker to hide and be reloaded.

Does the TOS-3 have GLONASS guided rockets?

I saw a recent video of a TOS attack along a tree line and the rocket impacts and explosions were very much closer together than usual... they seemed to land along the forested strip next to a road.... I suspect they are either guided or they have modified them to improve their accuracy.

The Smerch rockets had gyros to stabilise their flight and keep the rocket group together when firing at max range so they didn't spread out too far to be useless.

Not high tech or expensive... simple and sensible and cheap and made the volley of rockets land much closer together when fired at max range.

(note GLONASS guided rockets would be vulnerable to jamming, but gyrostabilised rockets don't use external sensors or signals for accuracy).

As an example if you added a gyro stabiliser to an RPG-7 rocket it would correct its habit of turning into the wind in flight...