They're called Totafot in the Torah, so why do we call them Tefilin?
Totafot is Hebrew Tefilin is Aramaic
Why there is no revival to call them Totafot, you'd have to be Gd to know why. Maybe because most of the people who like to bring back hebrew weren't themselves particularly religious. Or because Totafot only applies to the Tefilin shell Rosh... but still, nobody calls the tefilin shell rosh Totafot either.
In addition to Avi's answer, that "t'fillin" is the Aramaic targum of "totafot," in the Torah "totafot" is used to refer to the shel rosh, and "ot" refers to the shel yad.
In the Mishnah (and therefore in the Talmuds) "t'fillin" is used for both the shel yad and shel rosh; the singular is "t'fillah." My feeling is that the reason everyone calls them "t'fillin" nowadays is because everyone called them "tf'illin" back then, also.
Some sources for t'fillin in the Mishnah (not an exhaustive list!):
I would venture to say that since Tefilin was originally meant to be worn all day, which we do not do as we can not keep our D'aas on them, now we only wear it by Tefila, that is why it is called Tefilin.