Here's why:
* Aramaic was the common language of Judea at the time of Jesus. It was the everyday language of the people, even though Hebrew was still used in religious settings.
* The Gospels record Jesus speaking Aramaic. While the Gospels are written in Greek, they include phrases in Aramaic that Jesus is recorded as saying, such as "Eli, Eli, lema sabachthani?" (Mark 15:34) which translates to "My God, my God, why have you forsaken me?".
* Early Christian writings support Aramaic as the language of Jesus' prayers. The earliest Christian writings, which predate the Gospels, suggest that Jesus' followers prayed in Aramaic.
While Jesus would have understood Hebrew, it's highly likely that he prayed in Aramaic, the language of his people and his everyday life.