Companies tend to stick to their own ecosystems, and in this case Facebook's folly library has already bought into libevent, for better or worse. Hopefully once asio gets standardized more libraries will switch over, but until then there are a number of legitimate issues:
* There's a Boost version and a stand-alone version, you might want to use one but not the other:
* Boost requires linking against Boost.System, making it not header-only anymore.
* Stand-alone isn't as widely used and if you've already bought into Boost, you might not want an extra dependency.
* Asio's API documentation exists, but that's about the best you can say about it.
* The standard version is likely to resemble the library but won't be quite the same. When it gets standardized, you still have to adapt your code. One might reasonably argue it doesn't matter much whether to use one dependency vs. another if neither binary nor source compatibility can be preserved.
* The Networking TS didn't make the standard for C++17 so it'll still be a long time until the C++ world converges around it.
* Chris Kohlhoff's complete absence from both the Boost and stand-alone GitHub project's issue queues raises questions about the state of maintenance, there are lots of pull requests pending without upstream action. (The one email I sent him regarding his executors proposal also never got a reply.) Modifying their own library would be much easier for Facebook than getting asio patches upstream.
Personally I'd like to see a world where new C++ projects depend on asio, but it's understandable if others feel differently about it.