What would they gain by open sourcing it ? They have no incentive to get it to run on other platforms as it implements an open standard already in widespread use. In fact they have an incentive to provide a better browser on their platform only (not saying they are managing to do it, but they have an incentive to do that).
As for why they aren't using Blink/WebKit - I'm guessing they have huge amount of work based on IE and related tech and also integrating LGPL code in to Windows is probably a big no-no.
The fact that they're actively developing it and trying to best the open-source browsers in performance helps competition which is a good thing. Microsoft have actually continued to improve IE a lot since 9 in areas like web standards support which helps drive the web forward, particularly for technology-noobish people who don't install 3rd party browsers.
StatCounter's stats are very similar to Wikimedia's though. Net Applications is the outlier.
I've only seen IE in the #1 position in the stats of corporate extranet applications.
For all the regular websites it's generally Chrome and IE is either in 2nd or 3rd place. Firefox is kinda popular here, you see.
"Unreliable" is a pretty odd word choice, by the way. Global stats are always different from your own stats. By catering to a particular demographic you're inevitably introducing a massive bias.
Naturally, the only thing that matters are your own stats for a particular website. If it's 95% IE8 then that's how things are. IE8 has to work perfectly. There is no way around it.
Any of these interesting achievements are made somewhat ho-hum by the fact that there's no way to see what happened.