Hello @Practys,
The reason you can't buy BFP is because we do not own the game, it is owned by EA who closed it down in October 2013 but they still own the rights and IP if the project were to sell BFP or anything else in the game it would be against the law and also means the project would be closed down.
We are just a group of people who have all worked and continue to work in our spare time with no connection to EA and for free on this project with the sole purpose of bringing back a game we loved and so others could play it as well, this was sucessfully achieved on December 18th 2020 when full release occurred.
If you are looking to support us you can on our Patreon here where all the money raised there goes towards Forums and Server upkeep.