One of the most common complaints, or observations, about non-urban fantasy is that the genre is stuck in medieval technology. In part, I think this technology choice comes from the genre’s roots in medieval romances (Arthurian and others). It may also be the same impetus that draws people to ren faires, the SCA, and various LARP groups. That said, there’s certainly nothing wrong with upgrading the tech level of a fantasy world. Do you want secondary world Elves and Dwarves with muskets and airships? Go for it. But, there are also good reasons for technological development to be stunted.
Magic is the most obvious one. Some authors argue that magic and technology cannot co-exist, that they are inimical to each other. A great example is Ilona Andrews’ Kate Daniels series, where magic actively shuts down and tears apart the physical signs of technology. Alternately, there’s a good argument to be made for the presence of magic halting technological development. If magic is present and can be used to perform certain common tasks (such as transporting water or grinding flour) cheap or easily, then why develop aqueducts or windmills?
Disasters can also explain stunted tech development. Some sort of apocalyptic event, whether natural or magical, can set back or completely reset the development clock. The effects that apply in post-apocalypse literature, film, and TV can be applied to fantasy too.
For those worlds with multiple sentient species, racial mindset can also be a factor. A common trope with long loved or immortal species is the idea that they are slow to change. They may also be adverse to innovation. Whether Elves or Vampires, a long lived species could be so hidebound that their technological development is stunted. Depending on their population and influence, this could be a racial weakness or it could be imposed on other races.
Socio-political factors can also slow technological advancement. Two of the strongest elements here are religion and the upper class. Both have a vested interest in controlling technology and technological development. We need only look at writing technology in our own history to see this. For centuries, arguably even longer, only the priests, who’d developed writing, were literate. They controlled the technology of writing, which meant they wielded great power and earned riches by hiring our their services to the rest of society. Likewise, the development of the printing press was held back in both China and Europe. In the former case, the nobles purposely stunted development through laws that kept movable type from being invented. In Europe’s case, both the Church and nobles did what they could to limit printing presses, often in the name of combating heresy.