Gaps Remaining After Orthodontic Treatment

You wont get any better answers than what you read above this one. The only think I can think to add as a potential cause if YOUR TONGUE. Again, assuming you have all your adult teeth, your orthodontitst was thorough, you "occlusion" was solid and roots straight, no tooth size discrepancy, no additional funny jaw growth your tongue could be your answer.

You should google tongue thrust. The tongue is a very powerful muscle, working all day long, even while we are at rest. If your tongue is a factor a little bit of knowledge can eliminate it's influence and sometimes spaces will close with out further treatment.

There are many reasons why you might have gaps at the end of treatment or that might develop after treatment. Your teeth might be different sizes (the lower teeth wider than the upper so spaces can't be closed completely). Your jaws might be different sizes so that spaces might be necessary for uppers to overlap lower. The tip of the teeth also can affect spacing at the end of treatment. As for spaces that develop long after treatment, there is a tendency for crowding to develop in the lower arch and spacing in the upper as we get older. The bite settles, wears, and deepens causing these changes. If you have space that can be closed orthodontically, just remember that keeping them closed is a life-long pursuit. If you have spaces that cannot be closed orthodontically, having the spaces closed with veneers or buildups is a faster, more stable option. I did not have teeth removed as part of my orthodontic treatment 35 years ago and did not have spaces for the next 20 years. Recently however, spaces have appeared between some of my top teeth too (along with my eyes getting weaker and my hair thinner). I'm not crazy about the changes, but I realize they are normal.

Often there is a tooth sixe to arch size discrepancy in the anterior teeth.  This means that some of the teeth, often only  front four teeth are not large enough to fit the arch space (width of the jaw) that they have to cover.  When we know that the front four teeth are undersized relative to the space they need to occupy we usually have two choices - 1) leave a space behinf the lateral incisors as your picture seems to show or 2) plan to spread the teeth evenly in the areea and then close the spaces with veneers or bonding.  I prefer veneers over bonding.