It is tempting to believe that by writing a program for one platform, you can gain most of your market-share. However, people are using many platforms on many different CPU architectures: Windows 32-bit/64-bit on Intel machines, Itanium, or x86-64; Linux on a multitude of platform; BSD systems (NetBSD, FreeBSD, OpenBSD and others) on many architectures as well; Mac OS X; Sun Solaris (now also OpenSolaris), and more obscure (but still popular) Unix-clones like AIX, HP-UX, IRIX, SCO UNIX, Tru64 (formerly Digital Unix), etc. And to say nothing of more exotic, non-UNIX, non-Microsoft operating systems like Novell Netware, Digital Corp.s's VMS or OpenVMS, IBM's VM/CMS or OS/390 (MVS), BeOS, AmigaOS, Mac OS 9 or earlier, PalmOS, VxWorks, etc. etc.
As a general rule, the only thing that runs on top of all of these systems (in the modern "All the world is a VAX." world) is a C-based program or something that is C-hosted. [non-vax-like]Most good programs are portable to at least Windows and most UNIXes and potentially portable to other platforms.
For example, Subversion has made it a high priority to work properly on Windows. On the other hand many of its early alternatives, especially GNU Arch, could not work there due to their architectures. As a result, many mixed shops, Windows-only shops, or companies where some developers wanted to use Windows as their desktop OS, could not use Arch. So Arch has seen a very small penetration.
The bootstrapping C compiler of gcc for example, is written in very portable K&R C, so it can be compiled by any C compiler. Later on, this compiler can be used to compile most of the rest of the GCC compilers.
Compare that to many compilers for other languages that are written in the same language. For example, GHC - The Glasgow Haskell Compiler is written in itself, and requires a modern enough version of itself to compile itself. So you need to bootstrap several intermediate compilers to build it.
[non-vax-like] I'm fully aware that before C-based UNIX and UNIX-like systems became dominant there were some more exotic architectures that could not run C comfortably. Prime examples for them are the PDP-10 and the Lisp machines.
However, such more "unconventional" architectures are now dead, and no CPU architecture developer in their right mind would want to create a CPU that won't be able to run C and C-based UNIX-based or UNIX-like operating systems such as Linux. (Unless it's probably a relatively niche micro-processor for embedded systems).
Lisp, and similar higher-level languages, run on modern UNIX-based OSes very well, so there's not a big problem there.