Rugged, military and avionic systems are much more complex these days and building from COTS hardware is the only way to do this quickly and efficiently. In the past, COTS often meant more custom than off-the-shelf. There were standards such as slot size but the interfaces and software protocols were not set. This made it difficult to interface systems as well as provide upgrade or replacement paths as each new iteration required significant redesign.
The Sensor Open Systems Architecture (SOSA) is the current culmination of the move to standardize as much of the hardware and software framework to make it easier for vendors to provide hardware and software that is closer to what the final system will require thereby minimizing the amount of customization developers need to do to get the systems to work for their particular application. SOSA does not define new standards but chooses from existing standards like OpenVPX. It also selects subsets of standards to further refine the standards that systems will be built on. This allows vendors to concentrate on delivering systems that have fewer requirements as well as developers that have fewer choices making it easier to come up with an optimal configuration based on these standards.