Because of the Hindu Holocaust by Islamic Invaders. It was the world's largest systematic genocide that took place over centuries of waves of invasions:
There is no official estimate of the total death toll of Hindus at the hands of Islam. A first glance at important testimonies by Muslim chroniclers suggests that, over 13 centuries and a territory as vast as the Subcontinent, Muslim Holy Warriors easily killed more Hindus than the 6 million of the Holocaust. Ferishtha lists several occasions when the Bahmani sultans in central India (1347-1528) killed a hundred thousand Hindus, which they set as a minimum goal whenever they felt like "punishing" the Hindus; and they were only a third-rank provincial dynasty. The biggest slaughters took place during the raids of Mahmud Ghaznavi (ca. 1000 CE); during the actual conquest of North India by Mohammed Ghori and his lieutenants (1192 ff.); and under the Delhi Sultanate (1206-1526). The Moghuls (1526-1857), even Babar and Aurangzeb, were fairly restrained tyrants by comparison. Prof. K.S. Lal once estimated that the Indian population declined by 50 million under the Sultanate, but that would be hard to substantiate; research into the magnitude of the damage Islam did to India is yet to start in right earnest.
http://koenraadelst.bharatvani.org/articles/irin/genocide.html
The Muslim conquest of the Indian subcontinent led to widespread carnage because Muslims regarded the Hindus as infidels and therefore slaughtered and converted millions of Hindus. Will Durant argued in his 1935 book The Story of Civilization: Our Oriental Heritage (page 459):
“ The Mohammedan Conquest of India is probably the bloodiest story in history. It is a discouraging tale, for its evident moral is that civilization is a precarious thing, whose delicate complex of order and liberty, culture and peace may at any time be overthrown by barbarians invading from without or multiplying within.
http://en.wikipedia.org/wiki/Hindu_Holocaust
Afghan historian Khondamir notes that during one of the many repeated invasions on the city of Herat in western Afghanistan, which used to be part of the Hindu Shahiya kingdoms '1,500,000 residents perished.' 'Thus,' writes Vyas, 'it is evident that the mountain range was named as Hindu Kush as a reminder to the future Hindu generations of the slaughter and slavery of Hindus during the Moslem conquests.'
http://www.rediff.com/news/2003/oct/21franc.htm
It is tragic that thousands of educated and intelligent people seek to negate the Hindu Holocaust–mostly unwittingly. That is partly the result of reading fabricated history right from childhood, and partly for being politically correct. A nation cannot be built on a foundation of half truths and outright lies about its own history. It leads to mistrust among its own people, as is very evident today. There’s nothing shameful in admitting Islam’s destructive role in medieval India. On the contrary, it should, like the Holocaust museum, serve as a reminder of what should not be repeated. Ever.
http://www.sandeepweb.com/2006/11/09/hindu-holocaust-a-thorn-by-any-other-name/
In 1008 AD he captured Nagarkot (Kangra). The loot amounted to 70,000,000 dirhams in coins and 700,400 mans of gold and silver, besides plenty of precious stones and embroidered cloths. In 1011 AD he plundered Thanesar which was undefended, destroyed many temples, and broke a large number of idols. The chief idol, that of Chakraswamin, was taken to Ghazni and thrown into the public square for defilement under the feet of the faithful. According to Tarikh-i-Yamini of Utbi, Mahmud's secretary,
"The blood of the infidels flowed so copiously [at Thanesar] that the stream was discolored, notwithstanding its purity, and people were unable to drink it. The Sultan returned with plunder which is impossible to count. Praise he to Allah for the honor he bestows on Islam and Muslims."
http://www.hindunet.org/hindu_history/modern/moghal_atro.html