Because SDL 1.2 was EOL'd.

SDL 1.3 became SDL 2.0, which I will try to adapt to the Jolla device once it is my hands. Sadly, SDL 2.0's API compatibility with 1.2 was also broken, so it does not help SDL 1.2 programs.

As for the harmattan SDL, the 3D stuff should be quite generic, but the 2D acceleration (aka SDL_HWSURFACE) is both SGX, OMAP and "Maemo X11" specific. The reasoning is that I did not find another way to ensure v-sync in Harmattan without having to resort to a OpenGL surface which uses much more memory (and unlike Qt, etc. SDL 1.2 programs expect their "context" to be kept between focus changes, so dropping it at focus out was not an option).
I believe you should be able to remove the OMAP specific patches (pvr2d-*) without much conflict, but you'll lose 2D v-sync in the process.

Additionally, acelerometer, vibration, etc and all the other non-standard additions are also Harmattan specific, but these are less important. Namely, sensord for accelerometer, which is probably in Mer too, but Vibetonz for vibration, which probably is not in Mer.