There were a number of reasons. First, Finland was much smaller than Poland and not traditionally hostile to Russia (Russia could make a plausible case that it had freed the Finns from Swedish imperialism and left them better off). In fact, there was considerable precedent for this in the Russian Empire with the Baltic provinces, acquired from Sweden a century earlier, whose cities were allowed to govern themselves according to their traditional laws and whose elites were integrated into the Russian ruling class on very favorable terms. By contrast, Poland was much larger and was not only an independent state before the Partitions but was also a traditional enemy. There is a very plausible case that John LeDonne has made that Russia actually had no desire to partition Poland, knowing the problems it would cause, but was pressured into it by Prussia and Austria (and even then, in the First Partition it tried to take only Orthodox and Eastern Slavic language-speaking territories).

Second, the Finns were docile. Originally, after the creation of "Congress Poland" after the Napoleonic Wars, Finland and Poland were treated very similarly. Both countries received constitutions and were administratively distinct from the Russian Empire--a sort of personal union. It was only after the 1830 (and again after the 1863) uprising, which Russian ruling elites saw as a betrayal of the autonomy and freedoms granted to the Poles--after all, Polish people enjoyed privileges far in excess of what Russians had--that a serious crackdown on Polish autonomy and cultural independence began.

I think a third major reason is that unlike Poland, Finland was very much in the back yard of the Russian ruling classes, a short train ride away from St. Petersburg. Especially by the end of the 19th century, many Russian aristocrats had second houses in Finland and spent a lot of time there. There was not a whole lot of desire to rock the boat in the region since there was a lot of potential downside and not a whole lot of upside.

Finally, Finland was a buffer against Sweden, which after the final Russo-Swedish conflict in the 1800s was effectively out of the great-power game for good. (Actually, it was out in the 1790s, but Alexander took advantage of his brief alliance with Napoleon to grab Finland while the grabbing was good). Poland, by contrast, was next to two enormously powerful European rivals, and Russian elites believed that a firm grip was needed to protect the frontiers in the event of war. The resulting army deployment policy exacerbated the cycle of Russo-Polish discontent and crackdown.

Russia did not want finnish people to revolt and turn back into swedish kingom, thats about it. Treat newly conquered Finns nicely so they wont revolt.