I think the answer to this is that they were the first really major witch hunt in English history. We commonly think of witch hunting as being everywhere in the medieval and early modern period, but this wasn't the case at all in England. The numbers of people killed as witches in England was incredibly low compared to many places in mainland Europe, so when it did happen, it stood out.

Laws against witchcraft had only been brought in in the mid-16th century, and as far as I'm aware prior to the Pendle trials only one person in England, and one in Wales, had been executed for witchcraft, and in both these cases they were executed for murder by witchcraft, not just for practicing magic in itself.

A combination of factors made the Pendle trials possible despite the overall lack of interest in witch hunting in England. Firstly, Lancashire, where Pendle is located, was an area where the Reformation had largely failed to convert much of the local populace, meaning a great deal of suspicion from magistrates of secret gatherings of Catholics (who were more likely to be accused of dark magic and sinister conspiracies in England than witches were). Then, there is the ascension of James I to the throne in 1603. While the English generally had little interest in witches, he did, having conducted a witch trial in Scotland in 1590 and even written a book on the subject.

These factors increased interest in witches and resulted in the Pendle witch trials, which were not like anything that had previously happened in English history. Nothing like them would really happen again until the Essex witch hunts in the 1640s, which were slightly different in that they occured in the middle of a decade of civil war, in the context of a breakdown of normal law and order.

The Pendle witch trials also occured contemporaneously with another trial in Lancashire of the 'Samlesbury witches', who were all acquitted, which would likely have added to a contemporary sense that something unique was happening in Lancashire, and that the case of Pendle was special, given that their witches were actually convicted.