Many hold luggage scanning machines are automated with them being far more complex and sophisticated than the machines used to screen hand luggage, many of which are still solely reliant on the person who is monitoring the machine to decide what maybe a threat and what is not.
The weak link of the chain here seems to be the passenger screening process and by putting the potential threat into the hold, this takes away this weak link: the operator who is screening hand luggage.
The difference in robustness of passenger screening at many of these affected airports is very visible when compared to the likes of FRA and AMS etc. Perhaps this is why transit passengers are not affected, as they will be subject to what is deemed to be a more intensive body and hand luggage screening process during their transit.