DX9 was holding the development back, so they cut it loose: https://dolphin-emu.org/blog/2013/10/12/d3d9-why-its-not-part-dolphins-future/
The same thing happened for 32 bit x86 support recently: https://dolphin-emu.org/blog/2014/05/19/obituary-32bit/
There is an unofficial DX9 branch being maintained though: https://forums.dolphin-emu.org/Thread-unofficial-testing-needed-dx9-final
The same thing happened for 32 bit x86 support recently: https://dolphin-emu.org/blog/2014/05/19/obituary-32bit/
There is an unofficial DX9 branch being maintained though: https://forums.dolphin-emu.org/Thread-unofficial-testing-needed-dx9-final