Unmentioned reason: for the lone developer, the temptation to be "clever," or do interesting experiments is harder to resist. For me, sometimes that's driven by boredom. I've occasionally found myself switching paradigms ("This time I'm going to try a functional style (in python)...") just to keep things interesting, even if in the long run it might not be ideal. Even in the same project! Or, without the guardrails of peer criticism, I'll think "ok, this is a great time to try new library/framework X."

IMHO, there are some benefits to the above! Experimentation and trying new techniques/frameworks is how we learn and progress. The danger arises when the experiments congeal into half-baked production code.