Likely because no pressure is felt to put out a 1.0. The people that use it know what it is, and don't care that it's not 1.0 or above. It has constant, if low-velocity, development and bug-fixing going on[1] with 28 releases in over 20 years. PuTTY is a known quantity, I doubt anyone is trying to game the marketing to gain usage. For some people, that's a feature.
1: https://www.chiark.greenend.org.uk/~sgtatham/putty/changes.h...