Additionally airliners are designed for one purpose, they take off, climb to x thousand feet, cruise to destination, descend, land and complete cycle repeatably untill aircraft reaches its out of service date and the airlines buy another new one.
Military jets based on airliners, take off may or may not climb and cruise around and then land, they carry out manouvres that the airliner would not. Military aircraft also have extra bits bolted on, get re-engineered etc, change roles that mean the aircraft is doing what it was not 'originally designed to do', and the MOD/RAF do this not the civvie contractor.
As has been said any loss of life is un-acceptable and we cannot say often enough how we feel for those who lost family, but in the previous 30 years this has been the only loss of life from this aircraft attributable to an accident that was caused from a failure of this nature, the two other aircraft losses had nothing to do with the age of the aircraft, nor its AAR capability, nor its wiring, nor their lack of bomb bay suppressant system, nor the single skin fuel pipework, additionally I do not think the MR1? at Moray Firth had this type of problem as it does not have a bomb bay. This to me shows a pretty good safety record for the Nimrod MR2.
For all fuel/hydraulic leaks these have been risk managed and the engineers have done as much as they can, but once in the air vibration temperature fluctuations, turbulance and stress can all loosen pipes, cause cracks in pipes, which is why we have inspections and maintenance periods and we try to mitigate against these risks. As has been said, by looking at historical maintenance data, incidences and accidents, if a trend line shows that we have a specific problem with one specific area of a system, we put in an RTI, MOD, or other suggestion, it gets evaluated, costed and then the MOD/Government bean counters get involved, they way up the cost against likehood of catastrophic failure against possible effect (i.e. injury, fatalities loss of/or damage to aircraft and infrastructures), and as has been said if the risk of something happening is low, the effect of what happens if the failure actually happens is high or low and the cost is prohibitive then it will not get done, especially in todays RAF.