Skymap Pro 12 was originally a paid app (typically costing between $5 and $10). Its transition to a free model is generally attributed to the developer moving on to other projects or retiring from active development.
If you are looking for modern alternatives or related "Sky Map" applications currently available for free or as "Pro" versions, here are the primary options: skymap pro 12 free
Part of the KDE suite, KStars is highly underrated. Skymap Pro 12 was originally a paid app
Stellarium: This is the gold standard for free planetarium software. It offers a realistic 3D sky, telescope control, and a massive database. It is available for Windows, macOS, and Linux. skymap pro 12 free