It was looked at and rejected in favour of high efficiency solar cells. At the time of the design of Philae (early-mod 90s?) there were no European designs for an RTG nor any expertise in building them. If the Philae consortium wanted an RTG they would:
- Source it through the US -- you couldn't exact buy them off the shelf and have all the attendant ITAR baggage that would go with it. Since it would practically work as a US contribution to Philae there would be some science exchange in return. Not impossible, but at that time was there any US money to fund a further contribution to Rosetta from the US on what was quite a high-risk project? Not clear... It also goes right against one of the core principles of ESA which is to invest in/support European technology development.
- ... or fund the development of a European RTG; high risk and probably prohibitively costly for the money available to support the mission and meet the mass budget available for Philae. No doubt that mature designs probably do not have a huge mass penalty, but a new design? Who knows, or would want to take the risk?
- There was at the time quite considerable political resistance in certain European countries to RTGs in space. IIRC Germany was one of them and this would have put a big obstacle in the way. Development of solar panel technology was and still is considered an important goal and improved solar cell technology would be an important spin off.
In the end it really does come down to politics; the safety issues could have been mitigated (at some cost), but there was no political will to go in the direction of RTGs. It will be interesting what will be selected for JUICE...
Incidentally Rosetta itself suffers to a certain extent from choosing solar panels - the long array is turning Rosetta into a windmill that is quite difficult to steer. RTGS would have allowed a smaller array.