Note that none of the following is an official response. I am not a Roll20 representative. The reasons are questions for Wizards of the Coast. The same policy is being enacted on all digital platforms: Roll20, Fantasy Grounds, DnDBeyond, and is a decision that comes from WotC, the license holder. If you have bought the two soon-to-be-discontinued books before the kill date, you will continue to own them and have access to them, and so forth. You bought them. They are yours (as noted in the announcement above). If you have not purchased them before the kill date, the content will be unavailable for future purchase. The same will be true for any print copies, eventually, though physical stock will probably be available for purchase through booksellers until inventories deplete. My understanding from reading various message boards and announcements is that Wizards is now releasing many new campaign world books. The lore and some creatures in the discontinued books was specific to the Forgotten Realms. Future monsters in standalone books will likely be divorced from lore. Halflings in the Forgotten Realms are very, very different from halflings in Dark Sun or kender in Krynn. Eberron orcs are different from FG orcs, and so forth.