The best way to answer this question is to look at the parts of the Indian subcontinent that did become Islamized, namely parts of the northwest like West Panjab and Sindh (in what is now Pakistan) and East Bengal (what is now Bangladesh), and juxtapose them with the rest of the Indian subcontinent. When we look at these two areas, one of the things we notice is that both the territories of modern-day Pakistan and modern-day Bangladesh were relatively marginal and economically underdeveloped at the time of the Islamic conquests. Neither area had a substantial sedentary and urban civilization that could support powerful empires. All of the great North Indian empires were centered in the rich Gangetic plain, which included great cities like Pataliputra (Patna) and Kanyakubja (Kannauj), while Pakistan and Bangladesh had no urban centers of comparable size and influence until the rise of Lahore and Dhaka in much later times. Whatever states existed in Pakistan and Bangladesh were localized and restricted in scope, while polities based in the rich Gangetic plains (like those of the Guptas and Pratiharas) had far wider ambitions.
Because of their marginal nature, both Pakistan and Bangladesh could be characterized as "frontier" regions, where Muslims could (and did) make a big impact on the native population. In Bangladesh, for example, it was under the aegis of Muslim agents that large swathes of marshland were drained and brought under intensive cultivation for the first time in history; the Bengali colonists became Muslim, probably because the lords who pioneered this settling of the frontier (and thus became the local elites of the new sedentary society) were also Muslims, leading to many Bengalis being absorbed into the fold of this newly-made Muslim society. Similar events happened in Pakistan, where the local elites and landlords were overwhelmingly Muslim (including many foreigners like Pathans/Afghans), and they encouraged the settling of their landholdings by formerly nomadic or semi-nomadic Indo-Aryan tribes, who subsequently became Islamized.
In the rest of India, this Islamization did not happen for one of two reasons:
Muslims were never able to establish their firm political dominance in the first place, as in the case of Assam and Northeast India or the jungles of Chhattisgarh in central India.
The territories conquered by Muslims already had a well-established, sedentary civilization where almost everyone was already a part of Hindu caste society and under the authority of local Hindu lords (who were variously called rajas, zamindars, deshmukhs, etc.), and so there was little or no room for Muslim elites to absorb locals into the Islamic fold as they did in East Bengal or West Panjab, where frontier conditions predominated.