Because to the developers, Mame isn't about enhancements or running games quickly. It's about documenting how the arcade hardware works by emulating it, and to do that they try to make the code easy to understand rather than uber optimized. For example, a few games had Voodoo 3D cards in them. But Mame doesn't just pass on the 3D commands to 3D cards, even in Mame 1.07 which can use Direct3D; it emulates the Voodoo 3D cards in software, and thus runs those games extremely slowly.