Why have my ears gone back to how they were after having otoplasty?

You should discuss your concerns with the surgeon who carried out the surgery. Swelling means that it may be difficult to fully judge the result at 3 weeks. 
An important cause of early recurrence is a haematoma although these are usually obvious with swelling and blood discharging through the wound. The picture of the back of your left ear does not appear to show a haematoma. However, a proper medical assessment should be made of your ears in person by the person who carried out the surgery.
Reasons for recurrence may be a failure of communication as too how the patient wanted the ears positioned. The stitches that hold the ear cartilage in position cutting out. The natural springiness of the cartilage causing the ear to come back out. It varies with the technique used as to why the ear might come back out.
I do not thing the head band is important in stoping the ear coming out. I usually only advise people to wear the head band for 24 to 48 hours during the day and then at night for 6 weeks to stop the ear being pulled forward accidentally during sleep.
If once the swelling has settled you still feel that your ears are too prominent but you can not face another operation then you might consider 'Earfold'. This is a much smaller procedure than a full otoplasty but can be very effective in positioning the ear back. The great thing about Earfold is that we can show you how it will look in clinic using 'Prefold' which  is a similar clip that can be placed on the outside of your ear to simulate the effect of the procedure. The 'Earfold' clip goes under the skin and is permanent.
At present you should return to your surgeon and discuss your feelings about your ears with them.
Best wishes

A definitive judgement of the result cannot be made so shortly after the operation. Wait until at least 3 months have passed. Should a revision be necessary after this time, I recommend the MERCK stitch method. With this minimally invasive method, you can check and help determine the result in a hand mirror.

Without pre-op photos and better postop photos it is really difficult to tell what is going on.  It is possible for the sutures placed at the time of surgery to "pop" no matter how careful you may be.  But usually you feel that happening. Occasionally touch up surgeries need to be done to fine tune the result.Ultimately you will need to discuss your results with your surgeon and compare pre and post op photos, perhaps.  Sometimes patients have a harder time seeing the changes that others have no problem seeing, so your assessment of your result might be inaccurate (just be aware of this phenomenon).