Because a JSON which got exported from a Crate database is not in the standard JSON format, but a JSON object it needs to be imported in a different way than you would usually import a JSON.
library(jsonlite)
df <- stream_in(file("/folder/filename.json",open="r"), flatten=TRUE) # reads the file
write.csv(df, "/folder/newfilename.csv", row.names=TRUE) # saves the file to a csv format