Publishers have suspended support for the first generation Kindle. Whenever they issue a new e-book or update an existing one, this is another title that incompatible. Why is this happening? It has to do with the AZW format, publishers are no longer using it and this is the only e-book type that the original Kindle can read.
If you have an original Kindle and try and purchase a modern e-book from the last two or three years you will notice your Kindle is grayed out and flagged as incompatible with the book. Many people have pinged Amazon tech support and the canned answer is “it is a matter of incompatible format; that is, none of the books will work on the old Kindles, the solution is to buy a new Kindle.”
By flagging all books as ‘incompatible’ Amazon frees themselves from the cost of testing books for K1 compatibility and troubleshooting problems. The files may work on K1s, or they may not. Easier for Amazon just to assume they won’t work.