The PHP team is absolutely obsessed with backward compatibility. They broke surprisingly few things with PHP 7, and everything they broke was clearly documented.
You don't get to power a significant fraction of the world's most important websites if you break backward compatibility every few months.
They broke an unusual number of things in 7.1, a couple somewhat annoying e.g. defaulting DateTime('now') to including microseconds, without a single builtin function available that can strip/change microseconds on a datetime.
Yes, but they decided to keep it to 7.1, not 7.0, which meant the majority of the perf upgrades were available to most users who migrated to 7.0 _easily_.
7.1 was where all the breaking changes got pushed as a result.
You don't get to power a significant fraction of the world's most important websites if you break backward compatibility every few months.