Microsoft Windows as an operating system succeeded for a verity of reasons,
even though some of the competition on paper at least was better.
Windows was built on the back of MS-DOS. At a time when for every copy
of OS/2 sold (IBM's choice of operating system), Microsoft sold two hundred
copies of MS-DOS, so backward compatibility was going to be an advantage.
Remember IBM had defined MS-DOS as the de facto operating system.
Backward compatibility could still be a problem for Windows because
MS-DOS sometimes had restrictions like a 640K memory limit, whereas a built
from scratch operating system did not have this Legacy problem.
OS/2 could use 16 megabytes of memory (in fact it needed 4 megabytes to
run properly) but the memory crisis in the 1980's made software developers
nervous of writing programs for an operating system that needed very expensive
memory to run and then even more memory to run their programs, so development
of new programs for OS/2 declined.
Microsoft Windows 3.0 when released was not as powerful as a built
from scratch operating system but still had 90% of OS/2 features and could
run on a machine with an Intel 80286 processor, but was optimised
for the Intel 80386 processor, which was the processor Bill Gates rightly
expected to dominate the PC market into the 90's.
When released (May 1990) Windows 3.0 sold 3 million copies in the first
year. Microsoft then went on to write programmes that could take full advantage
of its operating system. This backward compatibility and the fact that
no one else had an application of choice for there operating system, gave
Microsoft Windows the edge. They have fought to maintain this edge ever
since.