Python 2 and Python 3 are effectively different languages in the way that VB and C# are different languages. Does it seem strange that some people code in VB when C# exists?
Python 3 isn't just a little bit incompatible in a few places, it has widespread incompatibilities and is therefore effectively a different language.
I dunno, it is kind of reasonable. C# was designed with a different mindset, but the intents were the same - take a predecessor (VB / Python2) and make them more modern, fix the gotchas and mindfuck, and produce something more refined that isn't backwards compatible.
C# takes it much further, and iterates on syntax a lot more, but Python3's breakage from Python2's is comparable I feel, just not in nearly as extreme a case.
Python 3 isn't just a little bit incompatible in a few places, it has widespread incompatibilities and is therefore effectively a different language.