Steampunk, before it became the full-blown sub-culture it is today, started out as a science-fiction sub-genre in the 1970s. The term is coined to describe a genre of speculative fiction in which steam, not electricity, drove technological advancements. It is all about mixing old and new: fusing the usability of modern technology with the design aesthetic and philosophy of the Victorian age.
This makes Wicca particularly well suited for use as a system of magic in steampunk fiction. It’s not the only historical system to be sure and systems like ceremonial magic and Freemasonry practice make for useful study for specific applications, but Wicca works very well as a general understanding of the ways and means of real world magical practice.