There is only few players on 32 bit systems, therefore its not worth making a 32 bit build anymore.
Like impossybull said:
Literally 99% of people play on the other 3 versions of SW. Almost all 32-bit systems are 15+ years old and can barely run SW at this point. Because of the old and poor hardware, many 32-bit users experience weird bugs in the Unity engine. Additionally, some 64-bit users will download the 32-bit version by accident and complain about bugs. You can build or buy a 5-10 years old 64-bit Windows or Linux system for even $100 that will run SW well for many more years. For these reasons the 32-bit Windows version of SW is discontinued.