Lots of library maintainers can't be arsed to put parentheses around their print statements (and perform the occasional slight change to other behaviors), or to run any of the automated tools that perform these compatibility-preserving changes. As a result, libraries maintained by these stubborn lazy people are not usable on python3, which makes beginners (who don't have a clear understanding of how these libraries are written and how easy it would be for them to perform the conversions themselves, because they still believe that "professionals" have some magic power other than the willingness to be persistent) wary of doing anything in python3.

It's very straightforward to write code that works in python 2 and python 3, and anyone learning python for the first time should be learning to write in that way. Ultimately, many of the compatibility changes are making things that have long been possible but considered poor style / bad habits no longer possible when doing so made the language easier to understand and behaviors more predictable (think print statement vs print function), or streamlining things that were originally bolted-on (think unicode).