Because the mobile web needed to be more screwed up
Saw this article this morning on The Register. Microsoft is unveiling a new mobile browser, Deepfish, which they plan will make viewing webpages on mobile devices more like being on a desktop. This will work by loading a "thumbnail" of the webpage and then allowing you to "zoom in" to the area you want to see. Supposedly Microsoft is saving on how much data will be downloaded by only loading the content you want to see. Perhaps I'm just too much of a standardista, but do we really need this? Microsoft claims this will alleviate the need for designers and developers to create a specific handheld stylesheet, and sure, it will, but isn't this getting away from the entire point of CSS and standards-based XHTML? Not to mention that this browser will only run on Microsoft powered PDAs and phones. The majority of cellphone users aren't using Windows Mobile devices, so what about them? They're screwed.
It's already bad enough that Pocket IE loads both the screen stylesheet and the handheld stylesheet. Then you get other devices like the Blackberry which broadcast that they are a desktop computer so they don't ever load the handheld stylesheet. Add into that the sordid state of built-in browsers on most phones with their poor CSS implementation and Opera Mini's built-in overrides and the mobile web is already a mess.
The Web is perhaps the greatest medium for content. It doesn't behave, however, like print and TV. For years, we as designers and developers have had to fight trying to force a square peg into a round hole. When big software companies do something like Deepfish, all we're doing is taking a miter to the round hole and slowly cutting it out into a square, when what we should be doing is finding a way to turn our square peg into a circle peg.

I like the design of the anglerfish.... reminds me of firefox's logo because of how it flows. but umm in regards to your post.. when doesnt microsoft like to throw a wrench in the gears and say "hey we are microsoft.. we tend to be a market leader so we are gonna do our own thing and force you to code in a way that suites US and then use your standards for those small little nobodies that are everyone else in the industry." Granted they arent really a market leader for Mobile devices but it seems like they are acting like it.
Let's all hope that Adobe gets apollo on mobile devices soon-then you can have an environment that runs webkit consistently across any models it's on.