Hi ardavang,
The "Import / export all data" function used to export to .csv, but has been changed to .json as we are unifying the export file protocol across platforms (i.e. using the same protocol for Windows and Mac). A .json file is simply another way of storing the data, and allows more information to be included when exporting/importing from Passwords - such as the tile color that was chosen for each login.
If you need to export to a .csv file, you can do so by clicking one of the "Export" buttons next to "Export to Google Chrome" or "Export to Firefox" - this will save the logins to a .csv file.
Please note that both the .csv and .json files store the login information in plain text. Please make sure to delete them as soon as you have finished using the files so that your login information is kept secure.