Space exploration was always very dependent on politics.

As Neil deGrasse Tyson says, NASA was founded on the fear factor of USSR's Sputnik which was launched using the intercontinental ballistic missile technology.

BTW: According to the comments here, the solar power generation on Philae weights at least 20 times less than the nuclear generator would.

It seems highly unlikely that any scientists would make subpar choices that may compromise a long-term project on the off chance that it might cause some political discomfort. The latter isn't even particularly likely since there is a large difference between nuclear power as an energy source on Earth and nuclear technology for scientific use in space. In addition most of the public doesn't even know or is interested in the technical details of a mission like this. Therefore it is extremely unlikely that using RTG technology would have caused any kind of political problems which makes avoiding said technology on political grounds simply nonsensical when it has the potential to compromise the overall mission.

that it wasn't a significant factor, at least