My understanding is that the Ottomans applied uniformly the Jizya all over their empire, yet in the Balkans the only people who converted were Bosnians and Albanias, all others clinging to their orthodox or catholic faith. Why them not others?
I don't have a great answer for this question, but I'll say that there are many other Muslim populations in the region, and there were even more before. Half a million Muslim in Greece were kicked out in a population exchange in with Turkey in 1923 (exempting only a small population of Muslims in Eastern Thrace) for a million or so Christians in Turkey. Bulgaria to this day is about 8% Muslim, but the overall percentage has been declining for decades. The Muslims are mainly ethnic Turks (who often have blue and green eyes rare in Anatolian Turks) but there are also Pomaks, who are ethnically Slavic Muslims and Romani ("Gypsy") Muslims. Albanians aren't just in Albania, but are in Kosovo and Macedonia and Macedonia also has a large ethnically Turkish population like Bulgaria. Furthermore, more than a million Muslims fled the Balkans between the 1880's the 1910s, and considering how much populations have grown since then (Turkey was 14 million people in 1927, and is 75 million today), they would make up a significant portion of Balkan population had they not left.
So, yes, Albanians and Bosnians were areas where conversion was particularly successful and I'm curious as well as to why, but it's important to remember they aren't the only areas historically with large numbers of Muslims. Some places, like Bulgaria, still have large numbers of Muslims, and other places had more but these were essentially ethnically cleansed as nation-states broke up the Ottoman Empire. It's also important to remember that often converting to Islam meant becoming an Ottoman (a Turk) so much of the "Turkish population" of Greece, Bulgaria, Macedonia, Romania, etc. was really local converts, not people coming from Anatolia (though there were also complicated population transfers of Muslims around the Empire, as well as voluntary migration).
A large part of the reason why Albania was Islamised was because of the clergy leaving the country amidst the Ottoman conquests. Many Catholic priests and bishops fled to Italy across the Adriatic and without any organised faith it was much easier for the Ottomans to begin a process of converting the peasantry in the country. A reason that contributes to both the Albanian and Bosnian conversions also was that both countries had significant religious turmoil prior to their conquests by the Ottomans - Albania and Bosnia alike had been the boundary between the Orthodox and Catholic Christian worlds, with Bosnia having the native Bogomilist 'Bosnian Church' to add to the mix.
That said, what yodatsracist said is also an important factor to include - even today, Muslims (mostly Turkish-speaking) make up 7.8% of the population of Bulgaria, and it is important to note that a significant number of Turkish people, especially those native to Western Anatolia, are of Greek ancestry, who were assimilated by the Ottomans from the 14th century onwards.
For Albania, there was never a real strong religious tradition there. Prior to the Ottoman conquest, the population was divided between Catholic and Orthodox, but wasn't followed very strongly. A factor for the mass conversion to Islam was it provided a lot more rights and freedoms than Christians were offered, with one specific thing being only Muslims were allowed to carry arms, and Albanians traditionally carried a ceremonial sword around. Even then they didn't really follow Islam, and in many cases retained their links to Christianity, with many examples of Albanians having both Christian and Islamic names, used interchangeably for the relevant purpose.
One theory, for Bosnia, is that it was because large part of population were neither Catholics nor Orthodox. Instead, they were members of heretic Bosnian Church. Bosnian Church lacked infrastructure such as places of worship, monasteries etc. Furthermore, it was already severely in decline even before the fall of the Bosnian Kingdom because of persecutions done by Bosnian kings who tried to gain support form the Vatican by associating themselves more with Catholicism. So, basically you have population that is consisted of mostly illiterate medieval peasants who lacked guidance of clergy. They had no churches to turn to or monasteries to write down and keep the religious practices. It was a flock without the shepherd, so to speak. On the other side, you have an organized religion backed up by strong and wealthy conqueror. Even then, the transition was gradual, lasting through several generations.
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