Thanks to their wholesale robbery of the subcontinent.
While predictions 40 years into the future(as if one can really put any faith on such predictions when the housing bubble was not predicted is another story altogether) might make this prediction that India, becoming the second largest economy in the world a surprising notion, for those who are aware of the histories of nations like India and China, this does not come as a surprise.
For example, India and China made up about 60% of the world GDP (around 30% each, +/- few percentage points) for nearly 1700 years of the past 2000 years. This all changed in the 1700s, with the advent of European colonialism. The graphs above clearly show how Colonial rule led to a direct and drastic decline in the GDP's of India and China..India in particular, as the British outlawed local manufacturing, took the factories to England, and made India into a land of raw materiel (cotton and Opium...btw the British policy of forcing local farmers to grow opium instead of Wheat(so they could make a killing selling Opium to China) resulted in untold Millions of deaths due to famine in India during British rule.) and end-product consumers for the British mills.
India, and China becoming considerably large economies in the future is but the reversion of the historical status-quo that existed for the vast majority of the last 2000 years. Some historians posit that the past few centuries might be looked back upon as a historical anomaly rather than the norm when historians in the future look at human history.
Here is an excellent DU thread on the Famines Caused by the Colonial rule, both under the East India company, and the later Crown rule.
http://www.democraticunderground.com/discuss/duboard.php?az=view_all&address=102x4534351