Because China got a 15 year head start on India.

Both countries used to have stunted, socialist economies that were roughly parallel in terms of GDP. Sometimes China would pull ahead, sometimes India. But the differences were small.

China began economic liberalization in 1978. India technically started in 1991, but most reforms didn't actually happen until 2001-2002. That's when the Indian economy took off.

Depending on how you count, India is 15 years behind China. India"s GDP at present is where China's GDP was in 2005, which is 12 years ago.

One difference is that China is ruled by an authoritarian regime which can impose reforms without much opposition. India is a noisy and chaotic democracy, so change requires consensus and comes more slowly.

However, if you look at growth before and after reforms, both countries are quite similar, it's just that China started a bit earlier.