The algorithm used by the science application doesn't have very many convenient "stopping places", where it has finished one section and is moving on to the next. When it reaches one of these points, it saves a checkpoint file that it can restart from.

In between these checkpoints the program has loads of very complicated information in memory, and it would require a very large save file, and lots of complicated code to restore the state of the program.

The Human Proteome Folding project checkpoints a lot more frequently. The grid techs are looking at options for checkpointing the FightAIDS@Home program more frequently, but they have more urgent tasks right now.