An interesting question. When someone asks this question, I consider it to be a very thoughtful question. I recently ran across this once again on Aron Gahagan's blog. I liked his answer about Jesus' two parents.
And I had this to offer...
Another reason could be because the four gospels present four views of Jesus. Two of the four require a genealogy and the other two do not. These views are analogous to Ezekiel's vision of the four living creatures.
"As for the likeness of their faces, they four had the face of a man, and the face of a lion, on the right side: and they four had the face of an ox on the left side; they four also had the face of an eagle." ~Ezek. 1:10
So, here we see represented a man, a lion, an ox and an eagle. And these four faces are seen in the four gospels.
The book Mathew certainly presents Jesus as royalty, a ruling King whose Kingdom is at hand, a Lion. As we know, royalty requires a royal pedigree and this is what Mathew gives us.
On the other hand, Mark presents Jesus as a suffering servant or even a slave. In this He is clearly the ox. Slaves have no pedigree, often they are not allowed a family name and therefore no genealogy.
Luke was a medical doctor. His writing reflects this since he presents Jesus as the man who is like us and understands our situation. All men have a genealogy Luke gives this to us for Jesus.
John, presents Jesus as the eternal one, the one with no beginning and no end; the eagle soaring in the heavens. Certainly there is no genealogy for one who is eternal.