Why Do Steroid Injections After Rhinoplasty Only Provide Temporary Relief?

Likely that Steroids are only artificially reducing your swelling. You have to let time do that.  if done incorrectly, it can actually cause atrophy so make sure that it is being done judiciously
 

Swelling
after rhinoplasty particularly in the nasal tip is normal and is a part of
normal healing. There is little to stop the tip of your nose swelling as only a
small volume of swelling can be visible in this area of the nose.

A
combination of factors will help with reducing swelling. Firstly, sleeping with
your head slightly elevated above the rest of your body. Secondly, the use of
taping compression of your nose soon after surgery. Thirdly, the use of
anti-inflammatory treatments to the tip of the nose, including steroids. Dr. De
Silva advocates the use of such medicine in most patients to speed up the
resolution of swelling and ensure faster recovery after surgery. Although the
final results of rhinoplasty can be more than one year after surgery, the use
of anti-inflammatories can shorten this period considerably. Patients with
thicker skin which includes certain ethnicities and ethnic rhinoplasty, may
benefit from more than one anti-inflammatory treatment.As with
every medication there is always a small risk of side effects and
anti-inflammatory medications are not suitable for every patient. Particularly
in patients requiring more complex revision rhinoplasty, caution is required as
steroids may rarely cause thinning of the skin resulting in a depression, or
loss of colour (termed hypo-pigmentation.

Steroids are an anti-inflammatory medicine aimed at temporarily reducing the swelling in your nose (or anywhere in your body). The results are only temporary because the underlying cause of the swelling (your rhinoplasty) will take many months to heal. The injections will give you a somewhat reliable preview of what your final result may be after your body properly reduces the inflammation in your nose over time. Unfortunately, all medicines have side effects and you must be cautious with steroid injections in the nose because over use can leads to permanent thinning of the skin, loss of pigment and or indentations in the nasal contour. 

Think of it as a nice preview. Healing and resolution of swelling after rhinoplasty is a biologic event that we are just unable to completely control. Time and patience are necessary.

 After Rhinoplasty, the nasal tissues swell and create internal scar tissue from the tissue dissection.  This is an ongoing procedure for months after a rhinoplasty.  Kenalog (steroid) injections temporarilly decrease this scar formation but eventually the steroid is removed, from the nasal tissues, by the body after which the effects would no longer impact the nasal tissue.