Most likely because of the 0.1N/kW. The best current made RTG's generate about 2.8 watts per kilogram. [source Wikipedia] Which means based on these current nu

Most likely because of the 0.1N/kW. The best current made RTG's generate about 2.8 watts per kilogram. [source Wikipedia] Which means based on these current numbers well need 3500 kilograms per Newton of thrusts. This would generate a acceleration of 0.00002 m/s^2 of acceleration. This would take about an hour, and a half to make a 1m/s delta-v course change. And that's without the drive, communication, heating, or scientific instruments. The drive is VERY low powered. Also specific impulse means little if you don't have fuel, since it determines your efficiency, but this rocket already has 100% efficiency since it has no on-board fuel.