I believe the ThumbGen developer got tired of people being ungrateful and borderline rude. Given the amount of time, skill, effort and love that must go into creating something like ThumbGen (and ThumbGen Designer,) giving it all away free is a major charitable donation to the users. But some users tend to become entitled and start with "Your program is not working, when are you going to fix it?" attitude instead of realizing that this is all a hobby for the developer costing him time with family or time that could be spent earning money.
After a while, such developers are likely to decide to withdraw (change user names, essentially) and keep any further improvements to themselves. I believe the current ThumbGen version is 6, though the publicly available version is 1.3.9.2