I think the answer must lie in practical issues - what means the relatives have to recover the fragments and what means they have to organise a funeral and / or transport the deceased to another country for burial. In Israel, strict Jewish medics search for every timey particle of a Jewish victim, to bring these together for burial. As the bomber is likely to be Palestinian, the relatives are much less likely to be given permission and time to do such a search, let alone leeway to transport the remains back to Palestinian areas, so unless parts are kept for identication then I guess the big bits are hygienically buried or cremated and the smaller bits hosed away.
If you take the matter of eg bombers in Pakistani mosques, the hatred of the bomber and their kin in the immediate locality is surely going to result in a similar train of events.