Motivation for projects can come and go, and is hard to predict much less control. One odd thing I noticed is that there was a feature I had been meaning to add to my WIP Atari 2600 game, and when I realized that it would actually be a pain in the rear to implement it, I suddenly had the motivation get it done. Unfortunately, that doesn't work with everything else in my life, however.
When I was with my ex wife, I did the cooking, and she tried a lot of different diets. I noticed that I enjoyed the challenge of making meals with the unique restrictions for each diet. Programming for retro platforms has a similar draw - making a game concept work with limited memory, graphical restrictions, etc. can be a fun challenge. I imagine most developers for these systems enjoy it for these reasons as much as nostalgia.