A synonym question is:

Why was the existing Windows not ported to the Intel i860?

In 1985 Windows 1.0 was released. AFAIK it had everything: Win32 API, DOS support, multi-tasking. The last OS of this production line was Windows ME in 2000.

Why did Microsoft create a whole new OS in 1993: Windows NT 3.1?
Which was roughly the same as Windows 3.1? (Win32 API/DOS support/multi-tasking)

Now the have to write all code twice.

On Wikipedia it says Windows NT was made for the i860. The Intel i860 is a RISC CPU. But I think the existing Windows kernel already had multi-tasking. So it would not be so hard to port it to a new architecture. For example the Linux kernel supports many different architectures.
Also it is not an argument because they later added x86 support to the NT kernel. So if it is possible to add x86 to an RISC kernel it should also be possible the other way around.

My first OS was Windows 98 so I never used these OSes. Please respect this...