A drone might hit the gun and damage it or they might try and hit the ammo, or they might hit the truck.
The truck can be replaced with a different truck as long as it can handle the weight of the gun, the gun can be repaired and ammo replaced.
The gun crew are spread out all over the place and it would be hard for one drone to kill them all.
Compare that with an artillery vehicle like a M109 where an anti armour suicide drone can hit the vehicle on the move or in firing position or anywhere in between and the ammo and fuel and crew are all packed into that tin most drones will be able to penetrate and when the ammo goes up the gun and ammo and fuel are all destroyed and the question is how many crew escape... bit hatches so most of the time they probably do.
The point is that a vehicle like Malva would not cost a lot more than a towed gun when you include the cost of the truck with the gun.
It can drive up to position but it can also use sophisticated electronics and laying equipment etc so it can pull up almost anywhere and drop its recoil spades and orient the gun ready to be loaded and fired.
Towed guns are getting more electronic too but being part of a truck mounted system you can have autorammers and other support features attached to the vehicle.
Right now it might have the MSTA gun on it so it shares the same ammo used in the battlefield by other friendly artillery units, while further down the track you could mount a different gun if you wanted... perhaps even a different calibre like some customers might want the old 180mm calibre long range guns.
Being able to drop ground supports, fire and then move... perhaps an ammo vehicle pulls up alongside and provides a few shells that are fired rapidly and then both vehicles drive away.
Less crew, less man handling, less digging the gun in to prepare it for firing, more stable platform faster in and out of battery... perhaps even a remote weapon station with a weapon on the cab roof that might be used to look out for drones or enemy troops.
Wheeled vehicles are not just cheaper to buy but vastly cheaper to operate... if you drive a MSTA 1,000km you would probably need a full check and perhaps a light overhaul of the running gear... a Malva could probably drive ten times further and be just fine.
You certainly don't want your entire arsenal of artillery to be on wheels but having a lot of them is affordable in terms of price and operational costs and crew numbers and the targets wont know the difference.