I will try to provide a brief response, at least with regards to German (Prussian) efforts, till someone more knowledgeable can perhaps provide a complete answer.

In short, by the late 18th century going into the 19th, suppressing the Poles was critical to Prussia's ability to (1) survive and (2) continue to grow, as an emerging European power. This was not the case to the same extent for the Austrians, whose territories had been part of Habsburgs' multi-ethnic lands for centuries and were indeed an established major player on the global stage. This inability to deal effectively with ethnic minorities will be one of the downfalls of the Austrian-Hungary empire come 1918. However, at this stage, between the capture of Silesia by Prussia (1742) and the partitioning of Poland (concluded in 1795), the Germanisation of Austria's minorities would not have been the pressing issue of the day. After all, the Austrian empire had included Italians, Croatians, Slovakians , Slovenians, Czechs , Dutch, Belgium amongst other nationalities for years. Prussia on the other hand, was for the most part homogenous (German majority) and much smaller in size so naturally there'd be more emphasis on incorporating the former Polish lands into the growing Prussian Kingdom.

What If Prussia was unable to "tame" its Polish gains ? what was to stop the Poles breaking away and reclaiming their independence ? or more likely be taken over by a great power ? This is where the survival factor comes into play; the Prussian Kingdom was located in a less-than-ideal neighbourhood surrounded by large competitors: the Holy Roman Empire (later Austria), the increasingly powerful Russian Empire and France (as Prussia gained new territories in the Rhineland). The map of Prussia between 1807-1871 highlights this vulnerable position: even after 1815 the kingdom was still not a single connected land-mass. This was something the Prussian rulers of this era were aware of and we can see this in their letters and testaments to their advisors or successors (i can't find the exact quote I hopped but i believe it was Fredrick The Great 1712-1780 who for example warned his successors of the precarious position of Prussian lands and that victories and gains can easily be reversed and lost - i.e. careful or you may lose everything we have worked for ! )

Of course any European power could in theory lose bits of territory, but the point is, Prussia could not afford to take such hits as a "new startup" whilst France and Austria as established major players could. (for a somewhat ironic example of this, see lands France lost to Prussia in 1870-71)

Consequently, Prussia had to ensure Poles were incorporated and Germanized as a matter of priority far more so than Austria at that stage. Else its newly gained territories (Poland and Silesia) may be snatched up by one of the Greater powers it was surrounded by and with it, her dreams of becoming a major European power and possibly her very survival (Duchy of Prussia anyone?) This wasn't just for prestige either to be clear; Prussia needed the resources, manpower and landmass these areas offered to compete with the major powers. The questions of size, population and resources only become more important as the Industrial Revolution kicked into full force.

Lastly, it should be noted the increasingly professional bureaucracy of Prussians as well as their strong dislike (hatred?) of Poles is likely to have contributed to how effective these measures were and how aggressively they were implemented. The numbers and tables provided here of how successful this Germanisation effort was, is quite telling.

As a side point, it's also worth pointing out the effectiveness of suppressive policy depends on the time period it was attempted. For example consider an attempt to suppress a minority culture before the ideas of the American or French Revolutions (and later Nationalism) had been understood and spread across Europe Vs an attempt, after, when these ideas had firmly taken root in peoples' collective consciousness in an increasingly literate and industrialising Europe in the mid to late 18th century.

And finally some speculation; it'll be interesting to know how much of Hitler's later hatred of Poles was inspired by Fredrick the Great's very negative attitude towards those people, in addition to other influences such as the pseudo science arguments of superior races etc given how much Hitler and the Nazi regime looked up to Fredrick.

Sources:

Iron Kingdom - the Rise and Downfall of Prussia 1600 - 1947 by Christopher Clark.

Foundations Of Western Civilization II: A History Of The Modern Western World by Robert O. Bucholz