There are some strong motives to do so. Anyone trying to float a reductionist theory of human behaviour tends to define human behaviour as being the result of simple and easily interpreted motivations, i.e. selfish motives. Altruism plays merry hob with that idea, because it suggests that people actually act for complex social motives rather than simple personal motives, which means in turn that you can't eliminate great swathes of social behaviour and social psychology from your model.
Since reductionist theories include many of the simpler neurophysiological models and behaviourist models, models that try and use simplistic Darwinian principles to define behaviour, most of economics, and several major strands of political theory, Altruism ends up with a lot of enemies.
I am reminded of a passage from the autobiography of Benjamin Franklin. He was a young man in London and went to visit a woman who was bedridden. She told him she spent all her waking hours in prayer, except when she had visitors or her Priest came to hear her confession. He asked to what she could possibly confess and she replied, "The closer my prayer brings me to God, the harder it becomes to resist feeling pride in myself."
There are no pure states in the human mind. History is full of stories of people who sacrificed their life so another person could live a little longer. If there is some reward for this kind of behavior, it is short lived. No matter what tiny satisfaction it might yield, it's hard to interpret such an act as selfish.