R. Tzvi Reisman has a long, in-depth discussion on this topic here, and the following are some of the main points which answer the question.
1) The obligation of this fast is unlike other fasts, in that it is only mentioned in a Baraissa of Mesechta Soferim, and not the gemara. Thus the obligation to fast started only as a minhag and so it is treated more leniently than other fasts. In addition, the gemara Yerushalmi seems to imply that one should not fast on Erev Pesach.
2) Several poskim explain that due to the weakness of our generation, if the firstborns fast on Erev Pesach they will not be able to prepare for the Festival, and may be too weak to fulfill the mitzvos of the Seder. Therefore, since this fast is already more lenient than other fasts, using a siyum to exempt the fast is the better option than fasting.
3) Another reason that some poskim mention is that there is an implicit understanding that a person does not intend to fast if there is going to be a siyum on the day that he intends to fast.