Every standard is intended to serve a slightly different purpose. Even if we're talking messaging standards over HTTP, you have both REST and SOAP. (and before I get all of the SOAP haters commenting ... there is a ton of bad SOAP implementations, but not everything is documented oriented and meshes well with REST)
Before we had SOAP there were standards like WDDX for encoding your structures as XML, then XML-RPC ... so standards evolve over time, and API providers may not change their systems to support the flavor of the week.
If you're working within a single discipline, there may be a set of standards, but again, as they each have a purpose, the odds of there being only one is quite slim unless that discipline's data is both homogeneous, and there's only one good way to search or visualize it.
For instance, in Astronomy, the International Virtual Observatory Alliance has over 40 standards. They all serve slightly different standards, for search query formation, serialization of results, etc. And that doesn't even include all of the work on FITS (Flexible Image Transport System, a file format with dozens of registered extensions) or WCS (World Coordinate System, used with FITS and other astronomy data)
I gave a talk at ASIS&T a few years back on standards and protocols in earth and space sciences and we're starting to recycle, converging, and otherwise simplifying things ... but it'll never completely converge, especially as people add new 'unifying' standards.