Why are some aircraft's life limited by both cycles and flight hours?
Some fatigue related issues that are not directly tied to departures and arrivals, like wing bending from gust loads, but may be randomly continuous throughout the flight, need an hour limit.  Taking it to a comical extreme, if the airplane flew for 3 years straight without ever landing because it had a new pixie dust powered engine, it would have close to 26000+ hours of bending fatigue time on the wings and tail, engine mounts etc., but only one flight cycle.  

Also, there will be an analysis that establishes the average amount of time per flight cycle, overall across the fleet, and this may be applied in some cases to give an hours life limit that is roughly equivalent to the target number of cycles, when applied in the aggregate.  That ratio of hours/cycles suggests that the average flight time per cycle for the PC-12 is somewhere around 1.1 hours.  In the Regional Jet world, the average flight cycle is about 1.3 hours.  The 30000 hour limit prevents a PC-12 operator who does a lot of long range flying with 2 hour+ trips from running his airframe up to 50000 hours before it gets to 27000 cycles.

30000 does't sound like a lot, but corporate aircraft generally live fairly pampered low-utilization lives and take quite a long time to get those hours, perhaps 30 to 40 years.  A regional jet gets to 30k in only 10 years.