These are crude estimates via calculations from Wikipedia, but 30% of British Indians are Sikhs and 33% of British Canadians are Sikhs. In the US, Sikhs are 5% and in Australia they’re 17%. Meanwhile Sikhs make up 1.7% of India. So even if my numbers are a bit off, Sikh diasporas are still proportionately way larger than in India. How did this come to be case?
There are multiple reasons for this. Sikhs have been a global community from their inception. Sikhs were spread across the Middle East into South East Asia during the community's development during the Sikhs Guru period. Records from the period note Sikhs from Baghdad to Chittagong, Myanmar and everything in between. It was also a multifaceted community with a mercantile class (Khatri's) leading the community, who were highly mobile trading communities. Sikh familiarity with the anglosphere predated the Sikh Empire, with individuals recorded in Salem, Massachusetts, by the 1760s. Many were already living under the rule of the British East India Company by the time Maharaja Ranjit Singh ruled over northern India from the 1800s. Ranjit Singh also incorporated many Europeans into his military and administrative ranks. Napoleon's officers (Ventura and Allard) commanded the Sikh armies; white Americans administered different regions while Scotsman, Germans and other groups were incorporated into army ranks. Meaning the Sikhs Empire was a highly diverse and prosperous society with people from all over the globe living, working and serving in the Empire. When the British finally annexed Punjab in the 1840s, they essentially took over a people highly adept to European ways and customs, trained under French Generals and very familiar with their counterparts who were now their rulers. Because of their prowess on the battlefield and willingness to travel, the British rapidly incorporated them into colonial machinery and sent them across the world. The military deployment, and parades, especially during the late nineteenth century, accelerated Sikh migration into the British Empire. Many brought back stories of new lands, economic opportunity and exciting adventures, which Sikhs took up in large numbers. Hindus had a lot of religious inhibition from travelling especially travelling over Kala Pani or Black Waters, which they interpreted as the ocean. They were much more apprehensive about leaving India initially.
Throughout the Indian migrations, Sikhs have always been at the top when it comes to leaving India, a lot has to do with this historical legacy and current contextual issues, such as politics, economics and the prestige of migrating to western nations.
Followup question: anti-Sikh riots happened in 1984 in India. Could there be a relation between this violent episode and migration of Sikhs to Anglosphere countries?