“Why”, has a long history of Microsoft constantly changing windows.
Initially anyone could use any choice of folder and ini files were either embedded in windows own directory or kept alongside the executable files. So MS suggested / required that 16bit and 32bit applications were collected together in Program Files, then along came more seperation for 32bit and 64…
Since the turn of this century, so as to save users from having an easy life, Microsoft introduced UAC security measures which mean it is no longer a Personal Computer thus now easier for applications to modify their settings in a range of im/personal folders such as %home% %appdatalocal% %appdata% and many others. Thus applications may install their data and exe files in a wide variety of locations. This often means there can be a handfull of folder locations each application is modifying.
If you use Adobe or Mozilla (Firefox) see how many different folders with those names you can find.
The developer of SumatraPDF decided it best for a single user to keep all the files in one location, (which previously was avoided except for portable version) and settled on a folder within the users Local Appdata rather than Roaming Appdata.
Local (Personal to the Machine User) rather than Roaming (Across domains) is a very moot point, which causes problems when a 3.1.2 user updates to 3.2 since the settings files are not generall migrated. However for a single user they generally use Microsofts licensed system as one person.
If you have more than one user, then there is an Administrator with enough elevated rights so as to install in Program Files. Then windows will allow multiple rights users to use the same installation, each with their own machine rights specific settings file in their Local Appdata.