I guess the most mundane explanation is that proper 'BIOS support' for this mode would have been too complicated. It's character based, so you'd need to write all-new versions of those BIOS graphics routines for setting pixels and printing text - not to mention the BASIC stuff for lines, ellipses, and flood fills. That could well have exhausted the allotted ROM space, but you'd also have to "snow-proof" those routines, what with the CGA RAM bandwidth issue in 80 column mode... which would've made them even slower than the original versions, if you can imagine that!

In hindsight of course, the BIOS functions didn't matter when you wanted any sort of performance, regardless of the video mode. But IBM will be IBM.