Trivial question, I know, but I’ve always kind of wondered about this. I mean, you don’t see references to the angels “Mike” and “Gabe,” or the other apostles “Phil” or “Matt” or “Andy,” so why the informal version of the name for Luke? Anyone know?
It isn’t a nickname for Lucas, at least not when referring to the Evangelist:
It’s just its own name.
Luke is the English language version of Lucas rather than a short form. In French language bibles he is Luc, in Italian Luca etc.
It’s the same reason you see, for instance, the bible refer to “Paul” instead of “Paulus”.
What they said. The major New Testament names were rendered into vernacular forms in the various languages of the Christian cultures quite early in the development of the national languages. The English “Luke” and French “Luc” may originate from a contraction of the Latin “Lucas”, but as Captain Amazing said, the same happened to Paul(us), it’s an evolution of the word, not a nickname.
My Koine Greek NT gives his name as Loukas, actually.
Because God foresaw the prequel trilogies and decreed that “Lucas’” name not appear anywhere in the Bible because of how wretchedly awful they would be. 
So, why no Johnathan?
And just as James is the English version of Jacob or Jacobus, and the original Semitic version of that name.
I think you may be on to something here.
Because it’s not an English name? There’s the name “John” and the name “Jonathan”, but not the name “Johnathan”.
Because it has become, for whatever historical reason, the standard English-language version of the name. It’s always bothered me that Marcus Antonius has been rendered in English as “Mark Anthony” or “Mark Antony”, when every other Roman named “Marcus” is always “Marcus”. It’s the vagaries of common usage.
So, “Tully” must really annoy you?
Interesting then that we use “Jesus”, a phonetic corruption of the Aramaic name “Yeshua” rather than using the anglified version “Joshua”
Referring to Cicero? I hadn’t come across it often enough to bug me, but if I had, it probably would.
I’ve been brought up on “Ovid” instead of “Ovidius” and other such commonplace shortenings, but , as I say, “Mark Anthony” is the only example (aside from the Evangelist*) of a “Marcus” being rendered “Mark”, and it bugs me.
*Speakin’ of which – why doesn’t this use of a common English form bug the OP? Or maybe they just didn’t think of it.
Could his be linked to Shakespeare’s Anthony & Cleopatra ? As a character he was well established in the English speaking consciousness early on with the Anglicized form o his name. Just a guess.