There doesn't seem to be any reliable information about why Frank Peretti, the Christian market author who radically expanded the horizon of Christian fiction, passed on writing Left Behind, that market's most successful series of books, the books that attracted public and academic attention to Christian fiction.
We know Tim LaHaye approached Peretti first with the idea for the series, and that Peretti passed, but why?
Peretti was, reportedly, LaHaye's first thought, but passed, and LaHaye found Jerry Jenkins, who at the time was known more for his children's series and his Christian athletes non-fiction, and we got the series we got.
In most cases, this is all the information given.
When a reason is given, so far as I've found, the reason is stated as fact without attribution. I haven't been able to find an instance where the reason given was clearly attributed to Peretti, LaHaye, or an editor or someone who would be expected to know. Maybe it's out there, but I haven't found it.
Some of the reasons aren't explicitly stated, either, but implied or just sort of waved at.
Plus, there are different reasons given by different sources. It's not the same one every time.
Explanations – including the implications and my extrapolations from sometimes cryptic explanations – fall out into three types:
Peretti is said to have questioned the feasibility of a series where the end would be known from the beginning. Where's the suspense? Alternatively, Peretti is said to have said that, after the rapture, wasn't the action supposed to be over? There isn't any room left for a plot.
Peretti was working on his own project. He released The Oath the same year as the first LaHaye/Jenkins book was released. Also, he doesn't like to collaborate. He did do a book with Ted Dekker in 2008, but, in interviews, sounds like he really didn't enjoy the process, and basically let Dekker take over and direct him, even though he was the senior partner. One could imagine him being very reticent to jump into a partnership with someone who was well established, especially if that partner’s job was basically going to be directing.
LaHaye is a Baptist. I think Southern Baptist, but that information is not readily available (his web site doesn't say, and the church he previously pastored presents itself as non-denominational, as does the college he founded, though there are reasons to think both are actually Southern Baptist affiliated). Peretti is Pentecostal, Assemblies of God. Though the theological difference between them apparently didn't concern LaHaye, it could possibly have been an issue for Peretti.