30,000 km (18.75K miles) is an easy interval for non-Americans to remember, and it is not far off of 15K miles. 20,000 km (12.5K miles) is also not far off of 15K miles, but in the other direction.
CRD has more torque, and that extra torque probably has a tendency to kill the fluid faster. It is kind of like towing. Towing something is rougher on everything. Extra torque is also rougher on everything. Thus, 18.75K miles is probably not good enough for CRD. The next lower convenience point would be 12.5K miles.
I bet that all non-CRD vehicles could easily go out to 18.75K miles on fluid swaps without issue. However, that is not the easy to remember fluid swap interval point, and you would not want to wait until 30K miles. 30K miles is the big swap point, and half of that is the semi-big swap point. Everything between is supposed to be small.
Manufacturers cater to the lowest common denominator. That would be the people who do not write any of the records down and only use their odometer reading to figure out what is due. I keep track of everything for all four of our cars in Excel. I do the math such that if I get "off schedule," I just stay off schedule, rather than do a change early when it is not needed to get me back on to schedule.
'06 XK Limited, 5.7L, QD II, Black, 139K
'02 WJ Overland, 4.7L HO, QD, Inferno Red, 109K