Because of who they were and who they were writing to. Good grief man, Luke says there were three languages used. Latin was the official government language of Rome, Greek was the language of the culture and Hebrew the language of the Jews. John even records who wrote it.
Let us examine that notion in some depth.
Matthew wrote his Gospel in Greek. He did write it for the Jews, but very few Jews of the area knew Hebrew - they spoke Aramaic. Why would he write a Gospel entirely in Greek and the inscription in Hebrew? Where does it say that, by the way?
Mark wrote to the Romans, but in Greek. If anyone would have written it in Latin it would have been him, but he did not.
Luke wrote to the Greeks, in Greek. Therefore it makes sense that he would have used Greek.
John wrote to the second generation of believers. Most of them were still Jewish, although an increasing number of them were Greek speaking. It makes no sense that he would write in Latin.
I think that looking for traps in my posts has kinda influenced the content of yours.
Hebrew was the language spoken by the ancient Israelites, and in which were composed nearly all of the books of the Old Testament. The name Hebrew as applied to the language is quite recent in Biblical usage, occurring for the first time in the Greek prologue of Ecclesiasticus, about 130 B.C.
In New-Testament usage the current Aramaic of the time is frequently called Hebrew (hebrais dialektos, Acts 21:40; 22:2; 26:14), not in the strict sense of the word, but because it was the dialect in use among the Jews of Palestine. [http://www.newadvent.org/cathen/07176a.htm]
Bottom line is that there were three different languages used on the plague above Jesus. There is no disagreement between the Matthew, Luke, and John. It was that each was giving what was written in three different languages.
Every time I am asked that question it arises from a faulty perception that the three disagree in what was written. The do not. You can try to obfuscate any which way you want but the fact remains that I know what the reason for asking the question was.