Why do you assume they did not know Greek?
Because they lived in a Aramaic speaking place. Hellenist were their enemies.
If you read the bible you would know this.
What is Galilean Aramaic? | The Aramaic New Testament
“After a little while the bystanders came up and said to Peter, ‘Certainly you are also one of them, for your accent betrays you.’” - Matthew 26:73
Because of how Galileans spoke differently, early Judean Rabbis thought poorly of them, accusing them of “sloppy speech.” There are several anecdotes in the Talmud Bavli (the “Babylonian Talmud”
where Galileans are mocked due to how they didn’t distinguish between certain consonants and vowels — sounds that were much more distinct and articulate in the prevalent Judean/Babylonian dialect. One such story even forbid Galileans from speaking in the Temple for fear that they might mispronounce something and offend God Himself!
However, despite these differences, after the fall of the Temple in 70 AD, there was a large migration of rabbis from Judea into Galilee, and that is when the dialect flourished. Great works such as Talmud Yerushalemi (the “Palestinian Talmud”
and the Rabba series of Jewish Biblical commentary were penned, and large schools were founded. The era of “Classical” Galilean (the “granddaughter dialect” to that which Jesus spoke) began and it continued into the Byzantine period.
Learn cultural anthropology so you don't keep making apologetically biased mistakes.