The Yamaha DX7 was the first popular FM (frequency modulation) synthesis keyboard. The FM sounds have been widely available (sometimes in scaled down form) in subsequent Yamaha synths. Smpled versions of the sounds are also widely available, but they lose the dynamic quality of FM synthesis. FM has given way largely to sample-based synthesis and, to a lesser extent, to physical modeling. However, there is always a market for old sounds, and if you want to sound FM, the best way is to use an FM synth. The old DX7 piano has approached the status of the Rhodes piano--they do not sound much like pianos, but they have been classic sounds in their own rights.

I think FM synthesis is very inexpensive to implement, so that is another reason it remains available.