To the best of my recollection, we've first flown a flyby mission to each planet we've visited before trying for an orbital mission. The reason is simple - the amount of propellant needed to insert into the target planet's orbit makes for a much more massive vehicle. Off hand, if they'd tried for a Pluto orbiter on this mission (and given up on the Kuiper object flybys), it probably would've had a liftoff mass about 2-3 times that of the current vehicle. That means it would take longer to get there because it wouldn't be going as fast after launch.

It isn't that hard to design a common satellite bus for Earth orbiting satellites. Hughes (now part of Boeing) has been doing it for at least 20 years. Doing that for planetary probes is harder due to the differences in environment (such as temperatures and radiation levels). If you design for the worst case such as the temperatures of Pluto and the radiation levels of Jupiter, you end up with a vehicle that isn't very well suited for anywhere. "One size fits all" works about as poorly for space vehicles as it does for clothing or education. There are some possible common components that might work at different planets, but the missions are flown so infrequently due to funding and planetary alignment constraints that they'd probably be obsolete before the next mission is launched.

Current missions don't need a lot of hand-holding like the old designs. It isn't like they'll have to be constantly monitoring it for the next 9+ years. After launch and checkout, they probably won't need to do more than the occassional state of health and track to see that everything is working properly. The technology for vehicles to navigate themselves and even automatically apply expert rules to systems malfunctions was demonstrated on DS-1 several years ago. Given the long propagation delays for this mission, I'd be very surprised if they didn't use some of that same technology on this one.