Because Hebrews dropped their 'ayins', to keep from saying God's name, hence we have "y'shua." Also spelled Yeshua, for which we have in Greek, "Iēsous" and "Isus." These are corruptions of the names that begin with "Yah."
Y’shua is Yahshua
After the return from Babylonian captivity the Jews began a practice of not speaking or writing God’s name, or His name in full. It is also known that the Galileans dropped their ayins, so as to not say God’s name. In a theophoric name even the letter yod can stand for the name Yah. Even today we use initial letters to represent a full name. Y’shua is Yahshua.