From experience, vehicles like the D-Max (and many others) are very hard on front wheel bearings. But you’re right, that’s really because the majority of them are all-wheel-drive while yours is rear-drive. So I would imagine wheel-bearing service intervals would be a lot longer than 30,000km.
The problem with four-wheel-drives is that the front wheels are being asked to do an awful lot. As well as steering the vehicle, they also have torque going to them from the engine. Also, since these vehicles tend to be used off-road, the wheels and bearings are routinely dunked in silty or even salty water. That shortens the lifespan of any components. The heavy turbo-diesel engine sitting over the front wheels also adds to wear in these cars, and could be part of the reason for such regular maintenance.
Or, it may simply be that Isuzu has made a blanket decision that all D-Maxes, regardless of driveline, have their wheel bearings repacked every 30,000km. It certainly won’t hurt to have the bearings inspected every 30,000km, but I’d be very surprised if they needed repacking every time. Some of these vehicles also use a sealed-for-life bearing and require replacement rather than repacking.
The difference would be if your vehicle was still under warranty (which, being a 2010 model, it’s not) at which point you’d need to stick with the factory service schedule to keep your warranty intact.