Why were the miners who looked for gold in California called the 49rs?

Miners were called 'forty-niners' because they moved to California en masse in 1849 as a result of the California Gold Rush.

Explanation:

1848 gold was discovered by James W. Marshall at Sutter's Mill, in Coloma, California. News of the discovery soon spread, resulting in some 300,000 men, women, and children coming to California from the rest of the United States and abroad. These early gold-seekers, called "forty-niners," traveled to California by sailing boat and in covered wagons across the continent, often facing substantial hardships on the trip. San Francisco grew from a small settlement to a boomtown, and roads, churches, schools and other towns were built throughout California. A system of laws and a government were created, leading to the admission of California as a state in 1850

because it was 1849 when the peak of the gold rush was (when everyone started to go to California)