It's easier for simple command line tools which just consist of a single source file (or generally projects so simple that they don't need a build system), and at the same time it doesn't add any overhead for integration into more complex projects compared to .h/.c pair. In general, more problems are moved from the build system into code (mainly the configuration, you can configure the implementation via defines in the source code in one place before including the implementation instead of passing them in via command line arguments (meaning more complexity in the build system), or a separate config.h header.

I tend to think of single-file libs as a "poor man's module system". The entire "module" is in a single file that you drop into your project, and you're ready to go (and ironically it's a lot more straightforward than the C++ module system).

The real-world differences between single-file and header/source pair aren't such a big issue as many people make it out to be though, either way is fine. The actual problem is libraries that consist of dozens or hundreds of source files and a complex build system setup.