One reason is that focussing on process instead of product is beneficial to fight procrastination. It’s much easier to work for 20 minutes on project x instead of finishing project x. So training habits instead of producing products makes it easier, mentally.
That means that a task list which mostly consists of product-related tasks (and most to do lists are like that) are overwhelming to us. On the other hand having a list of habits aligns nicely with our subconscious mind, since here the goal is to automate the process and get it out of our working memory system which can hold about 4 items at the same time.
Other reasons I can think of: todo lists grow stale quite quickly, project oriented todo lists don’t give context (e. g. where a task can actually be completed), a good todo list generally involves a lot of “metawork” to review it, keep it up to date, useful, effective, available, etc. Most (all?) ToDo-Lists are actually a terrible trusted system, because they are not meant to store and retrieve notes in a serendipitous fashion: todo lists are mostly meant to be worked through sequentially, whereas a good notes system helps being more intuitive/holistic. Structured text (basically all todo lists are trees/outlines) can make intuitive insights harder to discover: You tend to fit everything into a given project outline, rather than finding a new way to display the complexity of life.