The fact that the conversion rate on the Balkans during the Ottoman period remained pretty low has several explanations, but chiefly it was an economic issue. Christian subjects paid higher taxes than Muslims...If all the Balkan Christians has converted to Islam it would have created a huge deficit in the Ottoman state finances.
versus
The jizya was far smaller that zakat.
jizya = dhimmi tax on non-Muslims, zakat = tithing of Muslims