Reasons why IE, even IE9, is much harder to support than other browsers:

1. The IE tax:  Testing IE requires a VM license ($50-$100), plus another $100-$150 for a copy of windows. Plus a machine with enough RAM+disk to host the VM.  That's $250 out of pocket, just to test a single version of IE.  Nevermind trying to test versions 6, 7, 8,  and 9 (and, now, 10).

2. Inferior debug environment.  Yes, IE's debugger has (finally!) improved past the days of the Script Debugger.  But it's still  inferior to what other browsers offer.

3. Lack of automatic updates.  Often in other browsers one can simply wait a few weeks or month for a bug to get fixed and released.  Not the case with IE.  Yes, IE10 will have automatic updates (finally), but supporting IE today means supporting at least IE9 at a minimum.  And unfortunately that browser is still subject to the vagaries of user adoption, meaning developers will continue to have to code for the lowest common denominator feature set.

3.5  That lack of automatic updates is a killer.  Sure, IE9 has most of the standards support of other browsers today, but come back in 6 months and will that still be the case?  Or will it be standing still while Mozilla and WebKit race ahead with their standards development and support.  If so (and that's almost surely going to be the case), are you suggesting that web developers should commit to not taking advantage of these standards coming down the pipe simply so their sites will continue to support IE9 in the months ahead?

Your claim that Paydirt supports IE today, while technically true, is a pretty hollow argument.  Until IE10 is in production, and Microsoft is **aggressively** forcing users to upgrade to it, it's not going to change the decision-making landscape significantly.