There were multiple reasons based on my experience. New people were still using 2.7 tutorials because there was way more information out there than for 3. And package managers in Linux systems took a long time to change.
But the asyncio example is probably a better example of a fundamental program design change that's causing weird fracturing and incompatible library designs. Trying to use non-async from an async framework is a mess, trying to use async from a non-async is hard to integrate, etc.
I don't believe that python 2/3 change has much to do with sync/async issues. In python 2 we had twisted which was its own async universe. The sync and async worlds are disconnected in pretty much every language, but it's mostly a non-issue. You just don't mix them and it doesn't become a mess.
That's not what I said, they were two times that the community fractured.
There is fracturing with asyncio. Look at popular libraries. The "requests" library comes to mind, but there are others. It's hard to use different 3rd party components between the two paradigms which causes fragmentation in the libraries. You end up with aiohttp-requests and stuff.
PS: it does make a huge difference. If someone posts "how do I make a web server?" on a forum you'll get two different frameworks and approaches as answers. The community basically goes in two directions and much of the work becomes incompatible. The multithreaded sync vs event loop async just don't play well with each other. There still aren't many 3rd party libraries that really focus on asyncio because much of the community is still in the "threaded" design.
But the asyncio example is probably a better example of a fundamental program design change that's causing weird fracturing and incompatible library designs. Trying to use non-async from an async framework is a mess, trying to use async from a non-async is hard to integrate, etc.