It was born as a part of KDE Office Suite which hadn't wide usage. After community of Krita users grew rapidly (for an FOSS piece without marketing) it was separated from the bundle. Also a good boost was a publishing at Steam in the first month of their Software Store.
The Steam version is a nice way to sell open source software. It has an additional interface designed for tablets, which is open source, but the free builds from krita.org don't include it.
The current steam version is same as the free version, with steam version you get automatic update, The tablet version is Krita Gemini, i think it is the older version, i don't know if it is still on steam, as recently steam version was updated
