why were the nobel gasses considered to be inert gases until the 1960's?

Answer 1

Because no one had been able to make them react and form a compound until then.

Answer 2

Because before 1962 nobody was able to make any compounds containing the Noble Gases. Since then compounds of both Xe and Kr have been made, largely with fluorine, which is a very reactive substance (it has the highest electronegativity, therefore it is very good at stabilizing electrons, therefore even noble gases which don't really need their electrons stabilized will "put up" with fluorine bonding to them).

All these compounds are really sensitive and will instantly blow up in regular air, because the fluorine would much rather bind to almost anything else, and release the noble gas. So they're made under highly controlled conditions, and are practically not very useful, except to prove that even noble gases will react if you can force them to.

You can read more about the xenon compounds here:
http://en.wikipedia.org/wiki/Xenon#Compounds

Answer 3

Vincent is correct. They are tough, but not impossible, to make compounds with due to having a full complement of electrons for each level.