So, I have written about this before here and /u/marisacoulter has answered a similar question here and the short of it is that what's at play here is a certain measure of pragmatism, a measure of historically grown racism that varied between different peoples as object of that racism, and the question of an overarching anti-Slavism vs. a racism against Poles, Russians etc.

/u/marisacoulter in her answer cites John Connelly and his article Nazis and Slavs: From Racial Theory to Racist Practice in which he not only argues that "an erratic relation between ideology and practice" exists when it came to Slavs but Connelly goes as far as suggesting that no such thing as an overarching anti-Slavism existed and positions himself in favor or speaking of racism against individual Slavic peoples (Poles, Russians etc.) existed as distinct phenomenons from each other.

I'm not entirely convinced by Connelly's argument here because despite the wide-variety of policies applied to different Slavic peoples that ranged from massive atrocities against Poles and Russians to outright collaboration with Bulgarians, Slovakians, and Croats, Nazi ideology is still imbued with an overarching theme of Slav inferiority.

However, what needs to be taken into account is that Nazi ideology didn't just spring up from the ground fully formed but instead was something historically grown and reliant on ideological fore-bearers in its thinking. Taking that into account, it is noticeable that Nazi policy tended to be much harsher concerning people where there had been a history of antipathy and racism in German nationalist völkisch thinking. The cases where this is most notable is Poles, Russians, Czechs and Serbs.

The Nazis' collection of anti-Slavic prejudices originated from the attempt to explain how the world works and what the state of the world is through the lens of racial theory. Already in the 19th century (and probably earlier but that falls a bit outside my area of expertise), views of the Russian people and by extension in the 19th century, the Slavic people had a certain negative bend in Germany. Russia and its people were seen as backward peasants that missed the entry into modernity because of their "archaic" mindset, something amplified by the fact that the Russian Tsars in the second half of the 19th century took a viewpoint that was not entirely dissimilar and which in fact spawned certain reform attempts from up top (trying to abolish the last remnants of the feudal system in Russia e.g.).

Also, a strong presence within the German mindset about the Slavic people were the Poles. Large swaths of Poland were at that time part of German/Prussian territory and within the social set-up of it, the Poles occupied a socially inferior position with German / Prussian junkers owning land and Poles toiling it for them. This further shaped the perception of the Slavic people being a people / "race" that was predestined to serve its German masters as subservient.

The other important ideological strand, we can not neglect when talking about Nazi ideological perception of Slavic people is the Austrian situation since Hitler as well as other important Nazis were Austrian and strongly influenced by the political and ideological situation in the Austro-Hungarian monarchy at the time. The Austro-Hungarian monarchy throughout the second half of the 19th century and leading into the 20th century experienced strong conflicts based on a newly emergent nationalistic sentiment in its Slavic (Czech), South Slav (Slovenian, Croatian, and from 1878 on Bosnian), Hungarian, and German population with conflicts emerging surrounding the use of language, the question of political representation, and the German-speaking dominance of the state administration.

What really once again racketed up the level of the negative perception of Slavic people among the German-speakers of Austria and Germany was WWI. In the Austrian case, the pretty obvious negative stereotypes against the Serbs as well as against its own Slavic population that was suspected of harboring sympathies for the Serbian cause. The letter even lead to the Austrian government deporting hundreds of thousands of its own Slovenian and Croatian citizens away from their homes near the Southern border to cities like Linz and others and interning them there in camps. For the Germans, there was of course the old stereotypes of the backward and inferior Russians, which were heavily solidified by the experiences of German soldiers in Russia. Seeing the abject poverty many of the Russian subjects lived in while marching through their country gave a lot of people the impression that they were essentially a people living in filth and neglect.

With the Bolshevik revolution occurring in Russia in 1917, anti-Slavic sentiments among many of the racist early »völkisch« (racialist, viewing the world and history through the lens of a supposed race conflict) thinkers combined anti-Salvic and anti-Semitic sentiment with anti-Bolshevism. The outcome for them was the ideological formation that communism was the tool of »international Jewry« and the Slavs its expandable vanguard. This was highly influential for the Nazis. In essence, they saw the Jews as the puppet masters of international Bolshevism that sought to impose its rule through the »asiatic barabrity« and »eastern despotism« of the Slavic people. This Nazi propaganda poster gives you an impression of that. The depicted Commissar displays features attributed to Jews as well as Russians at the time and the depiction of the massacre below serves to portray their barbarity and cruelty.

Maria Toderova, a highly regarded scholar of South Eastern Europe, speaks in connection to the Western European perception of the Balkans of a phenomenon she dubs »Balkanism«. Balkanism is the othering (i.e. making the not like us in the discourse) of the inhabitants of the Balkans, seeing them as naturally »savage«, »violent«, »uncivilized«, »non-European«, »oriental« in a manner similar Edward Said described in his book on Orientalism for the Middle East. While the concept can not be transferred 1:1 on other Slavic peoples, it still rings true that in the Nazi -- and also wider German perception -- of the Slavs the latter were made out to be inferior, uncivilized, brutal, asiatic, and only fit as a »slave race«.

Prejudices and racism are social mechanisms that can be activated resp. employed in certain situations. When f.ex. Yugoslavia experienced the military coup that lead to it rescinding from the Axis treaty, blame was squarely placed at the feet of the Serbs who were attested a special tendency to treachery, argued with the murder of Franz Ferdinand in the World War. It was no coincidence that the Serbs took center stage here: While the Austro-Hungarian monarchy had not treated its Slovene and Croat citizens particularly well, the Serbs still held the place of enemy number one when it came to the South Slavs. Similarly, neither in Austria nor Germany, there was ever a pronounced and historically grown racism against Bulgarian people except in the most broadest terms. The fact that Croats had been subjects of the monarchy (and seen as less problematic as the Czechs) and that Bulgarians did never figure much at all in these calculations, made it easier for both the Nazis and the German population to justify a collaboration with them. It would have been much harder to be "pragmatic" about working with Poles f.ex.

So, in summary, the gist is that with Croats, Bulgarians, and Slovaks, the history of Germans and Austrian racism was different than in case of Poles, Russians, and Serbs, which made it easier to be pragmatic about working with them. The other thing that is important not to forget is that racism, as a system of thought that is based on something entirely ludicrous, neither is nor has to be internally consistent in what it purports and we can neither historically nor currently expect it to make sense, so to speak, because it's very premise makes no sense. With the Nazis as much as with other racists, racism is a mess of often conflicting assumptions about the other and about the world and therefore, consistency or indeed sense can not really be expected for like a conspiracy theory it retains a measure of ideological flexibility that makes it adaptable to changing circumstance since to its purveyors it needs to be maintained at all costs, circumstance be damned.