Why is Five nights at Freddy's 2 a prequel?

You can infer it from a few sources.

In FNaF 2, the date on your pay cheque is late 1987. It is implied that your character was the one injured in the 'bite of 87' which caused the venue to be closed. The CEO mentions they intend to reopen someday with a smaller budget, FNaF 1's venue is much smaller.

In FNaF 1, 'phone guy' mentions the animatronics have been 'singing for 20 years' - the venue opened in 1973 meaning it would be at least 1993, and he also mentions the daytime attack meaning it happened in the past. While it could be assumed that the animatronics in FNaF 2 are the scrapped models from FNaF, they could be completely different models (as there were other locations) or FNaF 2 is their first in-game appearnace and they were refurbished for the later reopening, as they appear in FNaF 1.