According to Wikipedia it was because,
... 'Due to metal shortages during World War I, iron replaced bronze between 1917 and 1919. Nickel-bronze replaced silver in the 10, 25 and 50 öre in 1920, with silver returning in 1927.
Metal shortages due to World War II again led to changes in the Swedish coinage. Between 1940 and 1947, the nickel-bronze 10, 25 and 50 öre were again issued. In 1942, iron again replaced bronze (until 1952) and the silver content of the other coins was reduced.'
https://en.wikipedia.org/wiki/Swedish_krona