I know what they should have done: They should have built the software with Silverlight, based on WPF/.NET 3.0 technology. Unlike "real" .NET, it exists also on Mac OS X.
But wait, that wasn't out 6 months ago...
Cross Platform development in the area of graphic intensive application is not easy. It requires extra a lot of extra work. Sure, you can use abstraction layers to ensure to "hide" the complexities of the graphics stuff from the rest of the application. But this doesn't mean that you don't have to to do the graphic stuff twice.
At my corporation, we are developing with C++ and QT, and we are also developing with .NET 3.0
The C++ QT application is currently working on Windows only. Overhauling it for Mac and Linux would be possible, but this requires a lot of work, for which we simply do not have the manpower. Maybe the guys doing the software will get around doing it in some time. But maybe not.
The .NET application is Windows only, since there are no implementations for .NET 3 for Linux or Mac yet. I hope this will improve due to projects like Mono and Olive, and also due to Silverlight. But honestly, after developing with .NET I will never want to go back. Maybe I'll have to, but I don't like to.
The realities of software development mean that there is a "design space" consisting of these variables:
1) Time
2) Money
3) Features you want.
if you decrease the time available for a project and want the same features, the money you require will go up.
If you decrease the money available for a project, it takes either longer or you will have to sacrifice some features.
If you want more features, the money will cost more or require more time.
Software Development fails these days (frighteningly often) because these rules are either ignored or certain estimates are simply wrong (sometimes intently to get the job, sometimes because people just don't know what they got into).
WotC knows that D&D 4th edition will hit the street on May 2008. Development started at some point before. WotC has probably only a certain budget reserved for the software.
So, to get the software in the time available and for the money required, it want probably this way:
"We need an application that will allow a graphic representation of a game table and a character, and allows the user to roll dice. We need it till May 2008 and it may cost no more than 900,000 $*"
Company A says: "Damn, we had to begin from the start. We'll get it to May 2008, but this will require our full staff and we might have to hire some external help. I guess that will cost 1,200,000"!
Company B says: "We have a 3D Engine half finished, so this will cut some time. But it only works on Windows. Oh, and it costs what you're willing to pay, and we will get it out in time".
Market Research: "Windows? Oh, that's pretty common. Would be better if it was universally used, but we figure this will make only 100,000 $ difference more per year"
WotC: "900,000 + 100,000 = 1,000,000. 1,000,000 < 1,200,000. So, for the first year, it will be cheaper than Company A? Company B, you got the job!"
*) all numbers made up with no knowing how the actual numbers would look like.
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All that said: I am a Windows and a Mac user. I love my new MacBook Pro, but there is really little software I need that runs on it. The MacBook is my "writing & watching movies" computer these days, which probably make it a waste of my money. But I like it anyway, and I would love it more would run on it.
I even bought and installed Windows Vista for it, though i am not using that too often. My old machine still runs the games I have fine...
oh, and as a general disclaimer: I always had a tendency for Microsoft OS (never got into Linux/Unix, despite having studied computer sciences and being a software developer), and since I am developing with .NET at my job, I became a kind of Microsoft "fanboy". So take everything I say with this in mind.

(Even though I must admit that there are certain features of Windows that could be a lot better. But Microsoft can build good OS - unfortunately, these are only research prototypes incompatible with everything we have these days.

)