This is because the Provinces nominate people that they believe will reside in, and contribute to their Province. If, for example, you move to Newfoundland, or Saskatchewan, and work the minimum amount of time to qualify to request Provincial nomination, and then that Province looks at your life and sees you studied in Ontario, and lived for years in Ontario, and you spent the shortest possible time in their Province before applying for nomination - they can reasonably ask if you actually plan to live in them long-term, or if you're just using them as an end-run around getting to live in Ontario. If you have strong links to one Province, and want to use the Provincial Nomination scheme of another, you're really going to have to show to the new one that you intend to stay there if they nominate you, and that their investment in you will pay off.
You don't have to commit to staying there forever, but you do have to convince them you're not using their Provincial nomination scheme fully intending to live in another Province.