I write about such software from time to time for magazines so I had a look upon them closely and I think, your guess is correct: NLE for video is easier than image editing.
See: the NLE in your list are first and foremost sophisticated remote controls for third party software, namely ffmpeg. Playing and transcoding of mediafiles based on XML schemes. That is solid work if done right (as in KDEnlive) but you dont need to write complex algorithms that can analyse pixel images in very very sophisticated ways and change the values of said pixels in even more sophisticated manner.
I have the highest respect for the people, who dare to take on projects like Gimp or Krita....

hugin is miscategorized with photo organizers; it is instead a specialized photo editor for turning multiple images into panoramas or HDR images