Islamisation of SEA follows the pre Islamic China-India maritime trading route spanning from Cham (Vietnam), Malay Peninsula, Sumatra, and Java, you'll notice that all of them became Muslims. Of course, one of the important things to note here is that after the sacking of Baghdad, the trading of sea-faring Arabian and Persian Muslims was more focused to the East. That being said, Muslims had a long history in SEA. They had presence in Canton as early as the 7th or 8th century, and that's where one of the oldest mosques outside of the Middle East is located. But they only gain foothold once trade over there intensified (lateen sails, compass and demand for spices contributed).

Muslims were the dominant force in the Indian Ocean trade, so they had a strong influence in the areas that partook in it. Local rulers and elites were attracted to Islam since conversion tied them into Muslim trade networks, won them the support of their influential Muslim populations and ideologically supported their autnomy from Hindu powers like Majapahit.

Additionally (to get back to the topic of Sumatra and Java specifically), as early as the end of the 13th century Indians from Gujarat (note the general rise of Islamic power in western India), through the Straits of Malacca, founded Muslim settlements in northern Sumatra; they are noted by Marco Polo. In 1292, Polo also observed that the kingdom of Perlak in north Sumatra had already gone over to Islam. These guys "used to be idolaters, but owing to the contact with Saracen merchants, who continually resort here in their ships, they have all been converted to the law of Mahomet." Partnership with Gujarati Muslims served as a clause for Islamization in Java as well. The wealth of these traders brought converts to Islam, and the influence of the religion becomes rapidly stronger after a Muslim sultanate is established in Malacca from 1445 when their king, Parameswaram, plus his entire court voluntarily converted, which was followed by the Chinese ruler of Palembang, a trading city. With the promise of trade and threat of war the surrounding people convert. Not just that, aside from wealth, conversion to Islam also ensured legitimacy -- great example of that is the Javanese Mataram dynasty getting the title of a sultan by sending a delegation to Mecca. So during the 15th and 16th centuries a Islam spread all over Sumatra and Java (and through the Malay peninsula).

Generally speaking, being under Islamic rule doesn't necessarily mean that the entirety of the population will eventually convert to Islam especially when the population is majority non-Muslim and when we are talking about a geographic entity the size of India. Especially when you consider that the Vijayanagara Empire defended the South of India for centuries. Instead of comparing Java and Sumatra to India as a whole, compare parts of India which had prolonged contact with the Islamic world or an incentive to mass convert. For example, you'll see trade as one of the main reasons of conversion in Kerala and Bihar.

Aside from the sheer size of India, you also have to consider political circumstances. For example, Delhi sultanate did go on occasional conversion sprees, but important thing to note is that was mostly when they were able to consolidate their power for longer periods of time. Tughlaq tried converting his subjects. But more often than not they were on a war footing trying to conquer more land, especially the then rich Rajput princes because of the overland trade routes they held. These campaigns resulted in general truces and occasional win for the sultanate, especially under Allauddin Khilji. Dynastic instability also plagued the sultanates. The nawabs of Bengal, on the other hand, were often in open rebellion against the sultanate, and didn't have a policy of aggressive conversion, but the imposition of jizya changed the religious map of Bengal.

The Mughals, however, initially barely had a toehold in India under Babur and Humayun, and were briefly expelled by Sher Shah Suri, AKA the guy who started the grand trunk road. The Mughals under Akbar consolidated their power primarily via a policy of religious tolerance. Of course, Aurangzeb violated this and we know how that ended for the Mughals. There were indeed mainly voluntary conversions, but they were prevalent in the northern parts of the subcontinent, like Pakistan, Kashmir, Punjab and north India in general.

A decent incentive to convert, on the other hand, goes a long way.