In the beginning, there were technical requirements
The pre-history of Swedish heavy self propelled artillery began in 1949 when Bofors was ordered by the Army to create a working prototype of a 155 mm gun for an SPG. This SPG had several names in documents: 15 cm kv fm/49, Akv 1949, and VK 152 S 49 (the last index was used internally within the company).
It was planned that the new SPG would weigh up to 30 tons and production of two units per month would begin in the spring of 1956. In the end, everything went differently: the production SPG entered service in 1967 and its mass grew to 52 tons. The increase in mass can be explained by the increasing need for more armour and protection of the fighting compartment and ammunition from weapons of mass destruction.
As for the significant delay, the problem here was the choice of an appropriate chassis. Additionally, as stated in a memo on April 1st, 1960, delays were caused by constant changes of requirements, a lack of qualified engineers, and doubt regarding the viability of the project as a whole.
Initially, three variants were considered from the tracked chassis: the existing Strv m/42 with a Volvo A8B engine supercharged to 450 hp and a VL 420 gearbox, and two more options, neither of which existed at the time. The first was an experimental project based on the suspension of the American M4 Sherman tank. The second was an unnamed future heavy tank. Perhaps this was a project by the Royal Administration of Military Factories (Kungliga Arméförvaltningens Tygavdelning, KAFT), later named EMIL, but it was a paper project at the time; its development began only in 1949.