I believe it's because his protective aura surrounds them, which is the same reason they need to be so tight.
His costume is made of fabric from Krypton.
...Krypton is the planet he's from. Not everything on Krypton is kryptonite.
Which wouldn't really make sense, as kryptonite is his weakness...
Since The Incredibles.
because he's SUPERman.
yep that was lame
Well that's justsuper.
In some of the comics his suit is made from the blankets that he was wrapped in when he was a baby, which were made from kryptonian fabrics. In others the suit gains invulnerability from constant contact with his body.