"They didn't" pretty easy answer. Also the idea that they lost "even though their products were technically superior to Microsoft's offerings" is sorta unsupported and is obviously subjective.
The primary advantage of the Borland's tools apart form itse its ease of use and was its very interesting component architecture which allowed for very easy development of third-party components and so many high quality third-party components were available free or at low cost. While this was happening MS got lost in the bushes trying to get ActiveX, COM etc to be the bridge for component inter-usability but it didn't come close to the ease of development of components in Delphi, CppBuilder etc.
The issue with Borland was the poor quality of management after their founder left. They tried to get into the Application Development Lifecycle space, bought up a lot of companies in that space and increased the cost of their dev tools to the point that it was no longer affordable to smaller dev shops who were their primary customers.
This problem hasn't gone away. Embarcadero still has ridiculous pricing for their products even though now they have a very stripped down IDE which can be used for free till you hit the USD 5000/year revenue limit.
MS design of their stuff in those years was often... shitty on multiple levels to be polite, ie MFC comes to mind, over-complicated for no good benefit. People jumped to literally anything else if they could, be it Borland for C/C++, Java had much saner object-oriented design model too (which could be compiled to native code with native UI if needed, since their default stuff didn't look the best).
I don't know about Mac support. I did Apple development during those days as well, but I didn't use Borland tools for that, I used CodeWarrior.
Are you sure “those days” were the same days? Turbo Pascal for Mac was released in 1986 (https://en.wikipedia.org/wiki/Turbo_Pascal#Turbo_Pascal_for_...), CodeWarrior in 1994 (https://en.wikipedia.org/wiki/CodeWarrior#Release). By that time, I think Borland already had left the Mac market.
Just left behind by the March of Java?
Just like Borland/Embarcadero, they now live from enterprises with deep pockets.
Back in the day, Visual C++ 6.0 was when we finally migrated into Microsoft development tools, and I used Borland tools for ages before that, starting with Turbo Basic in 1990.
Additionally to this day, Microsoft doesn't have anything on the C++ front that can compete with C++ Builder for RAD GUI development, MFC is a fossil, while WinUI with C++/WinRT is a bad joke.
He tells the story in this interview,
"Anders Hejlsberg: A craftsman of computer language"
https://behindthetech.libsynpro.com/001-anders-hejlsberg-a-c...
By the way, he also contributed to J++, that where P/Invoke, events, Windows Forms, properties came from initially (Yes, Delphi also had events and propertiers by then).
He certainly deserves credit for what he did, but not what those whose shoulders he stood on did.
Ironically 20 years later, Microsoft is again a Java vendor, and OpenJDK contributor.
If only IDL tooling, and related C++ code generation wasn't frozen in time, just like it first came in Visual Studio almost 30 years ago, besides updating the actual language to MIDL 3.0, that is.
And now while WinUI 3.0 folks tell the story that you can use C++ with WinUI / WinAppSDK, what they don't tell is the castastrophic state of C++/WinRT tooling in Visual Studio, that there are no plans to improve it past C++17, and the only thing happening ot its repo is bug fixes.
Stay away from it as much as possible, and using it with C# and CsWinRT is not much better, as many errors are surfaced as HRESULT exceptions, and you need to actually single step into C++/WinRT code to actually find out the real cause.