The simple (and more reasonable) answer: The FDA assesses the risk versus the benefits of a given treatment (in this case, prepulsid/cisapride) for a given illness(heartburn). The FDA also considers if there are more safer AND effective treatments available for that illness.
According to this article: http://www.ncbi.nlm.nih.gov/pmc/articles/PMC1173548/
, the side effects of this heartburn treatment include dangerous heart arrhythmias that may be fatal. Coupled with the fact that there are "countless medications" for heartburn that are safer and effective, the FDA (who "black box"-ed the drug when it first came out) has told gastroenterologists to use prepulsid/cisapride only as a drug of last resort.
You have to look at just more than the number of people who died from the side effects of a treatment. Given that heartburn is, for most, only a minor inconvenience that is easily remedied by reducing acid levels in the stomach, 70 deaths is high price to pay for heartburn relief.