I don't know why it was abandoned, it was before I even knew the project existed.
An OSD doesn't need to decode the video itself, it just needs the sync pulses. The EL1883 chip gives the cpu those pulses and it uses them to sync up overlaying what it wants on top of the video.
I'm told that the abandoned OP-OSD was used in creating the
http://brainfpv.com/ flight controller and osd board. So it lives on in a way...