Because of legacy. Spartan based on the old Trident engine, but refactored and without "compatibility" modes and quirks (as they promise). They need Trident to render old sites. Just imagine how huge is amount of old IE-only sites in corp. segment.
Completely true. But is it so hard to imagine a fallback instead of a rewrite / refactor?
I mean, aren't you tired to develop for X renderers? Couldn't Microsoft adopt something like the Khronos group for once and stop fucking around with proprietary bullshit?