This question's based on a false assumption. Gabriel and Raphael are common names.

I attended a very interesting lecture by Dr. Rory Cox of St. Andrews University on Angels a few weeks back. I believe he's already published several articles on the subject as well which might be worth a read if you're interested in the subject. His research was primarily on the divine war between Angels and Demons and the medieval studying of Angels but his research had some pretty broad reach on all things Angelic.

What I found most interesting, and probably most relevant here, is that the Archangel Michael became the Catholic St. Michael lending him an almost human quality. St. Michael is far more likely to have statues built of him and receive dedications since as a martial saint he could be called upon to lend victory in warfare. The ability of Archangels to interfere in the affairs of mortals was generally not accepted (certainly in the high middle ages) because it was argued that Angels were pure Form (in the Aristotle sense of the word) and had no matter. It turns out Angelology is complicated is what I learned. Anyway, this is a long way of saying that Michael was the most approachable of the three named Archangels which greatly helped his overall popularity in Catholicism. Gabriel and Raphael are hardly unknown as names for people, though.

We actually had a discussion about this in my World Religions class a few weeks ago. A lot of it has to do with Paradise Lost by John Milton. In Milton's epic poem, Michael commands the army of angels against the rebel forces of Satan. In the poem, Michael says, "Lend your ears to no leader but me." Many artforms have derived meaning from this poem.

I just sent an e-mail to my professor and I am going to see if I can get him to post here!

This isn't exactly a difficult question. Gabriel and Michael are the only named angels in the Bible (other than Lucifer, of course).

I'd say Gabe, Ralph, and Mike are pretty common names in America.