I can only speak for my personal experience - I write all my new code in Py3, and almost every still-developed library works great with Python 3.. The authors usually ported it several years ago. But sometimes you'll need something that's not still supported -

Perhaps you want to parse an old obscure file format, and the only code you find for it is from a usenet post in 1996. That code isn't being updated, and no one has ported it. That means you need to do the work to update it, and that can be hard when you aren't familiar with what it's supposed to do.

The other place I've seen people sticking with Py2 is when they've got a huge chunk of internal code. Some companies have been writing python for 20 years, and the original authors have long since left the company. It can be hard to write a business case for having someone spend several weeks updating all the old code, particularly if it's purely internal, and doesn't touch the internet.