Why is the “kol nidrei” Yom Kippur prayer said in Aramaic?

It was written by the Sanhedrin during the Babylonian exile when Aramaic was the daily language. There is a hebrew equivalent that many people say before a Beis Din (an informal one will do) of three people when they need to, rather than just before Yom Kippur, called 'Heter Nedarim'. It is interesting to note that the text of our Kol Nidre was altered from the original to refer to future vows rather than vows made in the past. This was done so that people who were forced to convert, but did not mean to but merely were making statements to save their lives, could do so more easily- knowing that these personal oaths to convert had already been annulled and were null and void. before the change was made- such people were effectively converting and until the next Yom Kippur were 'over an issur kares' (guilty of a sin with a punishment of kares) and thus if they died befor they had a chance to do heter nedarim or Kol Nidre they were in a very serious situation! By changing the text, the Rabbis prevented such a situation.

Many Jewish prayers are in Aramaic not just Kol Nidrei.

It was written during the time they spoke Aramaic