The Gospel in Mark

I've just started the book of Mark in my Through-the-Bible reading plan, and I've noticed something. In the first chapter of Mark, the word "gospel" has appeared 3 times-- in verses 1, 14, and 15. I didn't remember seeing that word too much in Matthew, so I did a word search to find out. Sure enough, the word "gospel" appears 4 times in Matthew, 1 time in Luke, 0 times in John... but 8 times in Mark. Considering that Mark is much shorter than the three other accounts of Jesus' life, that's surprising. Just to get an idea of the big difference here, if we divide the number of times the word "gospel" is used by the number of chapters in each of those 4 books, we get these fractions:

The word "gospel" is found in:
4/28 (14%) of Matthew's chapters,
1/24 (4%) of Luke's chapters,
0/21 (0%) of John's chapters,
but a whopping 8/16 (50%) of Mark's chapters!

To put this in even more perspective, in the whole New Testament only Romans, 2 Corinthians, and Galatians use the word "gospel" more times than Mark. Now, this doesn't mean that there's no good news in Matthew, Luke, and John, or that the saving message of redemption somehow isn't found there as much... but there does appear to be a special focus on the gospel in Mark. I'm not sure what the significance of that is, but it's something I'm going to be thinking about and looking for while I'm reading Mark these next two months.

Does anyone have any insight?