On one hand a lot of it is just not translated in English. You can find tons of books, articles and records on folklore and mythology in say Russian, Bulgarian, Serbian and so on. You can blame to an extend historians in the past 100 years, who tended to focus on the West or on Greece as the "craddle of western civilization".

On the other hand, there is the thing that the spread of writing and written language at least for Eastern and South-Eastern slavs was driven by the church and Christianization, and their focus was a lot more on establishing Christianity. Despite this we have some references here and there and a lot of knowledge surviving as folk beliefs (i.e. Vampires, samodivas), as rituals integrated in Christianity (i.e. In Bulgaria Sacrificing a lamb on st. Georges day or wearing martenitsi in march), or as beliefs connected to certain saints.

Follow up: Is there any current English-language literature on Slavic paganism?