While Java and Web have gone a long way to be cross-platform, there is much native software on Windows that has not been developed cross-platform. One such genre is design and engineering.
Computers are used for a large variety of tasks in the business world. I believe that the primary use is that of office document processing, e-mail, and web access. These 3 things, pretty much any computer can do. While the Mac does it very well, it is hard to justify spending $800 to $1000 when you can spend $500 on a cheap Windows PC (I'm talking with keyboard and monitor) - in bulk, or for a small business trying to make ends meet, I can see the arguments. It's a hard cost that is up front in their face when they write the check, and not a soft cost that comes by later in productivity or IT support.
However, for engineering software, there are limited choices. AutoDesk has introduced Inventor Fusion for the Mac, which works well and is a great 3D package. However, AutoDesk has not been taken seriously in many 3D industries like Automotive or Aerospace (where Dassault, Parametric Technologies and Siemens rule), and has struggled for acceptance in other industries like Heavy Industry and Manufacturing. They have made some inroads into Consumer Goods, but for the most part have had their strengths in 2D (mostly architecture).
Dassault Systemes has introduced DraftSight, a package that is designed to let users create and edit AutoCAD drawings - I have seen many AutoCAD (the AutoDesk 2D product) customers say they prefer to use DraftSight for its light-weight footprint, better performance, and refreshing take on User Interface. This is available on the Mac, but I wonder how many people have downloaded it for that platform.
What I see as a big hole, and perhaps a big opportunity, is the lack of high-end, premium 3D design packages for the Mac. And nowadays, a 3D design package is only as good as its back-end data management system (now called Product Lifecycle Management, or PLM, system) - so not only does the package have to allow designers to draw 3D geometry, but it has to interface with the PLM servers. So it seems that a significant effort is needed to get a design package to the Macintosh platform.
As an aside, you could imagine that Apple, Inc. has a need to design their products to build (digitally, not by drafting board!). And, for those designs, they would be made in 3D. That begs the question, what software does Apple use? I am not familiar with electrical design software (especially electronics, micro-circuitry, and chip design), but there again the Windows platform has the solutions the industry uses. Apple designs and builds many of their chips, and so must use Windows to design their Ax chips. I have looked into that via web searches, and have found some rumors that sound credible. It appears to be a secret, but I would bet that Apple is using Windows-based 3D software, and it seems even more believable that they are not using it on non-Apple PC's (i.e. they are booting Windows on their Macs). Now, it would seem that being able to do full 3D design in a Mac-native platform would appeal to them - but of course, Apple is insanely profitable, and doesn't care about paying Windows licensing fees. But maybe they do care about performance and productivity, and my experience has been that once people use OS X, by and large they tend to want to stick with it primarily, and avoid Windows like the plague.