Coastal Dalmatia and Zagreb were not conquered as it did not lay in the route to Vienna. Not necessarily important if you have Hungary. Slavonia however was on the road to Buda and Vienna which made sense taking it.
I can see the Ottomans go for Croatia. Especially if the Ottomans really want to go for Italy, that is like Ravenna and Ancona. It offers support from Bosnia and Croatia to Italian campaigns.
The Ottomans were not really interested in Hungary. Odds are Hungary would remain independent if it weren't for the Habsburgs trying to take it. And when Zapolya failed to defend against Ferdinand the Ottomans took initiative and conquered Central Hungary. Otherwise the best the Ottomans prepared to take was key fortifications around the Danube and closer to home: Zemun, Petrovaradin and just maybe Osijek. As a defence for the Balkan. But no more. A vassal Hungary was good. A vassal Hungary without Croatia? Even better.
A bit of clarification and an interesting tidbit:
Before the Ottoman conquest the capital of Slavonia was Zagreb and most of Slavonia at the time remained under Habsburg control. Croatia was only the territory next to the adriatic - most of which has fallen to the Ottomans. It was only during the ottoman period, when the habsburg were organizing the parts of royal Hungary still under their control to captaincis (sorry I cant find the correct english term for this military administrativ units, in hungarian they are: főkapitányságok, the whole of Hungary has been organised into six of them) that the border of Croatia was extended north to the river Sava. Zagreb was still the capital of Slavonia. After the reconquest of Hungary a miliary frontier was organized all along the border. The territories in it were outside of the civil administration. As most of the former Croatia became part of this Croatia was moved once again to the north and this was the time when its capital became Zagreb. Slavonia was moved east along the river Drava. So when you say the ottomans conquered Slavonia you are mistaken, as the Ottomans only conquered what later became Slavonia however at the time was not part of it.
One of differences between Croatia and Slavonia was that Slavonia has been part of Hungary from its founding. The bishopric of Zagreb was founded by the hungarian king László I. when Croatia was still an independent staate. An important result later was that slavonian county's were represented at the hungarian pairlament and the croatians were not - this remained in effect till the 19th century.