This declension is called Attic because in other dialects, including Ionic and Koine, the nouns are declined normally.
In Proto-Greek, Attic-declension nouns had long ᾱ ā and digamma (ϝ w) before the endings.
The Doric dialect preserved the ᾱ, but lost the digamma by the classical period.
In Attic, η was shortened to ε e and, if possible, the vowel of the ending was lengthened to ω ō or (if it was a diphthong with iota) ῳ ōi.
The shortening and lengthening was caused by quantitative metathesis, the switching of vowel lengths.