Why is English the most common language spoken in commomwealth?

The premise of your question is not accurate. While English is widely used in many Commonwealth nations, it is not the most common language spoken overall. Each Commonwealth country has its own primary language or languages, and the languages spoken vary widely across the Commonwealth. For example, Hindi is the most commonly spoken language in India, French is the official language of Canada and several African nations, and Spanish is widely used in Caribbean Commonwealth countries.