The goodly part of the answer lies in the partitions of Poland. As one can see in the map, Russia absorbed a great deal of the departed Polish kingdom. Now, examine the boundaries of the Duchy, as seen in this map. Although the Duchy absorbed territory from each of the partitioners (Austria, Russia, and Prussia), there was significant room for expansion and the fear in St. Petersburg was that any expansion of the duchy to a regal status would come at the expense of Russia's Polish gains. Napoleon's elevation of Central European princes and electors to kings set a precedent as they often absorbed their smaller German neighbors, but there were other factors at play that would have ensured that an aggrandized Polish title would need more territory. Poland had been without a king for over a generation and any figure Napoleon elevated to royal status such as Prince Józef Poniatowski, who Napoleon later claimed on St. Helena would have received the Polish throne, or Marshal Davout, who some in the imperial court felt angled for the position, would have to use Polish nationalism as a means to cement the loyalty of the people. The other areas of the Partitions were much less suited for Polish expansion according to the national principle. The remaining areas of the Austrian and Prussian partitions were demographically quite mixed, with cities like Poznan being fairly Germanized. Elevating the status of the polity from a geographically delimited area, Warsaw, to the more expansive "Poland" was a signal that Napoleon was seeking to redraw the political boundaries in Russia's front yard, perhaps not only creating a Kingdom of Poland, but also a Duchy of Lithuania as well.

Even if the elevation to kingdom status did not involve a major expansion of territory, one of the palpable fears for Alexander II I and his advisors was that by rekindling the Polish national spirit they would not only have problems in their own partition territory with irridentism, but that it would make it much harder for Russia to make its political weight felt in European affairs. The Duchy lay athwart the main route for Russian arms if they were to make their presence felt in Central Europe and a Polish client state acted as a buffer that would prevent the Russian army from a quick march. This was part of the reason why the Duchy was such a disruptive element in Eastern European politics in this period as it acted as a French check on the partitioners. The Duchy was not strong enough to act on its own, but it threatened a two front war on both Austria and Prussia if they moved against France in Central Europe.

The irony this opposition is that while Alexander II I fought against the Kingdom in war, during peace he became a champion of for the restoration of the Kingdom in peace. Congress Poland would have a dynast in the form of the Romanov tsar and existed as a personal union with him. The rationale for this reverse course was much the same as before. Alexander II I could pose as a champion of Polish national rights if it came to war in Central Europe and harness Polish national feeling for their lost territories in Prussia and Galicia. That was the theory anyway. In practice, the Russian state's honeymoon with Congress Poland did not last past Alexander II I and Polish irridentism and Russian suspicion of Polish nationalism became one of the consistent facets of Russian rule over Poland for the remainder of the empire.

So if you've played Crusader Kings you might remember that holding the title of a duchy or kingdom can give you claims on land that historically 'should' belong to that entity. This is historically accurate. If the Duchy of Warsaw became the Kingdom of Poland, it would be justified in expanding its influence into large portions of western Belarus and Ukraine (which at the time were held by the Russian Empire), whereas the Duchy of Warsaw would not. This doesn't mean Russian land couldn't be given to a Duchy of Warsaw, but it would cause more political upheaval.

After that, you come to the issue of where the Kingdom of Poland even ends. The last time it existed was during the days of the Polish-Lithuanian Commonwealth, but the Kingdom of Poland technically wasn't the ruler of most of that territory. That right belonged to the Grand Duchy of Lithuania, which at its height controlled lands from Vilnius to Kharkov whereas the eastern border of Poland was somewhere around Lviv or Halych. If it could be reasonably justified that the Kingdom of Poland was the successor state to the entire Polish-Lithuanian Commonwealth and/or the Grand Duchy of Lithuania, the only logical Russian response is "holy shit" because Napoleon can now annex nearly all of Ukraine and massive amounts of developed western Russia into this new state. In fact, this wouldn't have been that much of a challenge considering that Poland had only been dissolved 20 years previously and thus was within the memory of most people.

This can be compared to the Polish-Soviet War a century later where Josef Pilduski attempted to essentially recreate the Commonwealth after pushing far into Russia (see Neal Ascherson's The Struggles for Poland - it's actually a really interesting concept). His problem was that the age of multi-ethnic empires had just recently passed and revanchism for Poland-Lithuania had all but dissapeared. Instead, Poland became a strange, semi-multi-ethnic semi-nation-state with a very weak national identity which perfectly contrasted with the ultranationalism of the rising German Reich to its west. But I'm going off on a weird tangent here.

Another very important matter is one of prestige. The Duchy of Warsaw was a de facto client state of the French Empire under Napoleon. It would be a most prestigious act to hold at your whim such a massive empire - especially one which used to be of such historical importance. As a plus, Napoleon might have even won the support of the Polish people for resurrecting their nation and their nationalism despite living in a client state. Although under French sovereignty, the Duchy of Warsaw (and therefore the projected Kingdom of Poland) was much more autonomous than any Polish community under Prussia/Austria/Russia.