Most languages simply don't make gratuitous breaking changes to things people rely on in most of their programming. The only other cases I can think of in a major language would be 1) Visual Basic, where Visual Basic .NET is really a fundamentally different language that happens to share some syntax conventions, and so got the moniker "Visual Fred" in the VB6 community, which was pretty pissed about the whole thing... but it is also true that other than in branding, Microsoft never really pretended Visual Fred was anything more than a way to make old-time Visual Basic developers a little more comfortable being forced to write C#-ish code... and 2) Perl 6, which left copious development effort and active maintainers on the runtime engine for Perl 5, which continued to receive reasonable updates (so no one was stranded, like they were with Python).

Here's a non-example: Ruby. It was quite possible and not even that difficult to write code that ran on both Ruby 1.8 and Ruby 1.9 that had correct behavior for both systems, despite all of the pain people noted about the transition with Unicode just trying to use vanilla 1.8 code on 1.9... but the transition from Python 2.7 to Python 3.0 required an all-or-nothing approach for the numerous changes which could not be "from __future__ import"ed in 2.7: Python 3.0 almost seemed to go out of its way to screw with developers, renaming "str" to "bytes" and "unicode" to "str" without providing any aliases which could be used across the different runtimes and simultaneously removing the u"" prefix syntax while remapping "" (so there was no cross-version way to even say "this represents text). Even later versions of Python still didn't try to help with exceptions: the syntax simply changed and the only way to write code that worked on both was to catch exceptions without a name and then dig into sys.exc_info() to pull the value as part of the except block.

Meanwhile, the Python developers handled the bootstrap horribly: they have been giving the finger to people for using Python 2 ever since they started working on Python 3, putting 2.7 in what was essentially an "emergency maintenance only" mode, causing the runtime to stagnate for five years while they marched from 3.0 (which was nearly unusable) to 3.5 (which was where Python 3 started to look reasonable) and there was very little library uptake; so it isn't like higher-level code could transition even if it wanted to: application developers were stuck waiting on libraries and library developers were stuck waiting on the language itself to become more reasonable, and in that five year gap many people just started jumping ship to entirely different languages--such as Go, Rust, Clojure, and Elixir--as it isn't like they really had a choice (as again: they were blocked on people who were blocked on other people).