So when Audacity 1.0 was released, new feature development moved to the 1.1 branch, while the 1.0 branch received only bug fixes. When the 1.1 branch was considered "finished" it became the new stable version 1.2.0, and the 1.0 branch was abandoned. Then new feature development moved to the 1.3 branch, which has now been released as the new stable version Audacity 2.0.
If I remember right, many of the major changes between the 1.1/1.2 series and the 1.3/2.0 series were under the hood, such as support for new versions of Portaudio and wxWidgets (which added compatibility for newer hardware and operating systems), improvements to the file format, and enhancements to the importers.
Since there's no "right" answer for version numbers, it's hard to answer questions like this. If the developers feel that 2.0 is a good number for their new release, then who are we to argue with them? (And why should we care in the first place about what number appears in an about box somewhere..?)
It's been eight years since Audacity 1.2 was released (and at least six years since 1.3 development began), and the code has seen pretty major changes in that time even though they happened gradually and not all are obvious from looking at the UI. I think it's fair to call 2.0 a major upgrade from 1.0 and 1.2.
Or to look at it another way: If you haven't changed the major version number after ten years of development, then you'll probably never change it. And if you're never going to change that number, it's redundant and you might as well drop it. (This is roughly what the Linux kernel did when it switched from 2.6.x.y to 3.x.y after fifteen years of 2.x releases.)
Audacity uses even/odd version numbering for stable/unstable release branches, like the Linux kernel once did: https://en.wikipedia.org/wiki/Software_versioning#Odd-number...
So when Audacity 1.0 was released, new feature development moved to the 1.1 branch, while the 1.0 branch received only bug fixes. When the 1.1 branch was considered "finished" it became the new stable version 1.2.0, and the 1.0 branch was abandoned. Then new feature development moved to the 1.3 branch, which has now been released as the new stable version Audacity 2.0.
If I remember right, many of the major changes between the 1.1/1.2 series and the 1.3/2.0 series were under the hood, such as support for new versions of Portaudio and wxWidgets (which added compatibility for newer hardware and operating systems), improvements to the file format, and enhancements to the importers.