What made Islam so attractive to the West African, East African, and Austronesian rulers who converted?

That is a very wide area, and people to discuss such a specific topic. In West, East Africa and South East Asia people had many different reasons for why they converted, there is no one silver bullet reason for why they did.

Two underscoring reasons come down to trade and Sufi missionaries as reasons.

Trade, because through traditional trade routes across the Sahara or down the Indian Ocean and across to Indonesia, Muslim traders would setup a small mosque for themselves (with permission of the local ruler), and take on families in those different places (whom they obviously converted). Generally speaking, it could be commercially advantageous if you converted to Islam, to be included in this community of Muslim traders.

Further to the point, Islam is an "egalitarian" religion, in the sense that everyone is equal before the God and religious law, which was a very popular concept, much like Christianity was popular in the Roman world. If you had been a Hindu slave/untouchable or a merchant, people who were of very low caste; by converting to Islam you could free yourself from these societal bonds, and become equal within the umma (Muslim religious world). Islam was the only religious which had an expressed wish, to see its followers commit once in their life to pilgrimage in Mecca; which was a practice which picked up steam with the Mamlukes (in Egypt), after the deposing of the Caliph. Pilgrimage was not only a spiritual endeavor, but also a mercantile venture, to meet new people and make business contacts.

Now around the 9th century (very roughly), you began to have Sufi orders spring out of the Kharijite rebellion in Algeria. Sufi's were missionaries and mystics, who setup lodges around the world for their supporters. More pertinently however Sufi's in spreading Islam, were happy to make accommodations to local religions, in a syncretic mix. So if you went to West Africa, or East Africa, they were happy to mix local deities and traditions, with Islam, to create a local blend (much like Christianity's spread).

And finally, and far more generally, when a ruler converted to Islam, often for political/economic reasons, as well as I'm sure for spiritual reasons, his subjects would often convert overtime, as patronage circles changed, and Islam became sponsored by the state.

If you have any questions let me know, this was a brief summary of a very large topic. I have a variety of sources, but I would encourage you to look up Richard Buillet (Columbia Professor Emeritus), who has written a lot on the topic of conversion to Islam. He also has an uploaded lecture series on youtube/itunes, where he discusses these topics.