Reason 8.2 and later requires a 64-bit operating system to run. By dropping 32-bit support, we can focus our work on more modern operating systems and architectures that allow us to spend more time developing features and improve the software.
For occasions when you still need to run Reason in a 32-bit environment, for example if you have a ReWire host that only runs in 32-bit mode, version 8.1 is still available for download here - provided that you have a Reason 8 license on your account. Note that Reason 8.3 and 8.1 can co-exist on your computer since they are kept in separate program folders.
Note that Reason 8 is no longer supported, so you do this at your own risk.