Historically, many infrastructures were based on vCenter for Windows, especially those that were not deployed after vSphere 6.5 was announced. Before vSphere 6.5 was introduced, the vCSA was an appliance with a limited set of features and many companies preferred to use a full-feature vCenter Server running on Windows.
From vSphere 6.5, vCSA is the preferred vCenter deployment option, and vSphere 6.7 is the last release that supports vCenter for Windows. Customers are encouraged to perform the migration from vCenter for Windows to vCSA.
VMware introduced the vCenter for Windows to vCSA migration wizard in vSphere 6.5 as an integrated part of the vCSA installation.