Microsoft has many faults, but backwards compatibility is not one of them.
I doubt this would be the reason. (Also, TrueCrypt runs on OSX and Linux too, so a build environment dependent on Windows-only seems odd).
There are several things MS has released that don't install on newer systems very well. There are some where the installer depends on an old version of Internet Explorer being installed and which fail miserably on newer versions.
So if you have a Windows-based build server, it's probably running Windows Server 2008 or later. Which is 64-bit only.
You can run it on a recent 32-bit version of Windows, but client editions only.
And in the next year or two I imagine Windows 9 or 10 going exclusively x64/ARM.
I will try to reinstall it today, and have VS 2008 running inside.
Align this with what the TC website says now: "development of TrueCrypt was ended in 5/2014 after Microsoft terminated support of Windows XP."
Could it be that the original developer is somehow unable to update the build process to work on newer OSes, or unwilling to do so? Maybe they don't trust any VC++ released after 1993, and that version is probably not going to work on Windows 7 or 8.
[1] http://www.infoworld.com/t/encryption/sloppy-secure-open-sou...