The main problem with the 15 cm cannon wasn't the gun itself, but the ammunition. Kinetic penetrators (KP) was considered a thing of the past given the rapid development of the HEAT-technology, which was the natural choice for a brand new tank system (hence the large calibre, as the power of the HEAT-round is a direct function of the diameter) to be built.
HEAT-rounds have some problems though:
* In order to detonate properly at a target, the fuse must be exact enough to ignite the hollow charge at a precise distance from the armour. This is getting increasingly difficult with higher projectile speeds, so the Vo (velocity measured when the projectile just clears the muzzle) deliberately had to be kept low (500-800 m/s) for the fuses to function.
* This in turn means that for longer distances, the trajectory becomes much higher than for a faster projectile, meaning the flight time becomes much longer than the mere difference in Vo between a KP and a HEAT-round suggest. Long flight times makes it a lot harder to hit a moving target...which everyone playing derp-gun and artillery can confirm.
* Finally, the HEAT-rounds (just like HESH) doesn't like getting spun around, or rather, the centripetal forces will lower the penetration. This causes severe problems in a gun relying on spinning the shell for stabilisation.
The Swedish military naturally wanted a reliable shell with good penetration, precision and short flight time at a reasonable cost per shell.
These simple demands proved to be insurmountable for the Bofors engineers. If the flight time was to be kept short, the velocity had to increase, which made the fuses unreliable. Getting good penetration with a rifled gun was difficult due to the spin, and with a smooth bore gun the lack of stabilisation made the precision suffer. Improving the fuses and adding a ball-bearing allowing the outer casing to rotate around the inner HEAT-charge and stabilise the projectile while giving full penetration would have made the shells prohibitorily expensive (15-50 times the cost of a normal HEAT.shell) to stock for war time use, never mind exercises.
Without a working shell, the gun couldn't go into final design, nor could the auto-loading system. Those in turn were the determining factor for the final design of the turret, so hence it was never made. Bofors could of course have made a more realistic mock-up, but that would just have been a waste of resources as all machinery would have to be re-tooled once the final designs were in.
In short, this is the kingdom lost due to the lost horse-shoe nail.