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The application binary interface (ABI) that a C++ compiler is written to is much more complicated than that for a C compiler. In C++ the ABI must deal with not just stack layout and allocation of simple objects, but with many additional issues, such as:
Every time a new language feature is added, the existing ABI mechanisms must either be contorted to support the new feature, or the interface must be changed or extended. To take the former approach for the large number of features being accommodated would severely compromise the robustness and maintainability of the compiler, not to mention the performance of the generated code. Thus, binary compatibility with previous versions is sacrificed instead.
The SCO compiler is not unique in this regard; all other C++ compilers have had to break their ABI one or more times in upgrading to the new standard as well.