You can clarify this one, but frankly there are huge number of political leaders opposed to the Communist Party in Poland. In fact, many of the key individuals within the Polish government were not on the flight, such as the more powerful Prime Minister as I previously noted. (Even when you throw in the members of the Sejim, with the a total of 460 members and people with similar views likely to get elected the benefit isn't there.) Getting rid of a few critics is clearly not worth the risks involved, while you'd be looking at very petty revenge as the other possible motivation while Russia would putting itself at risk of genuinely basically becoming an international pariah state if they were caught causing the crash. (Its not even like the flight was confined to politicians and military leadership, but also included journalists and a variety of other civilians.)