(1) People find it dull because it is taught badly. In fact, the Synoptic Problem is often not taught at all. In so far as New Testament introductions and introductory courses teach it, they focus on one particular solution (the Two-Source theory) and then they refract all the data through that theory. Simply setting out a solution deprives students of all the interest in the process of history, all the enjoyment in puzzling out the literary enigma.
(2) People find it dull because they do not actually study it. The only way to engage in serious study of the Synoptic Problem is to get down and dirty with the Synopsis, and to spend enjoyable time working with the texts, ideally doing some colouring. It's one of the guilty secrets of the guild that too many scholars simply do not do the work with the texts that they should, preferring instead to keep wading through the pile of largely mediocre pieces of secondary literature.
(3) People find it dull because they think that there is an obvious solution (the Two-Source Theory). Alternatives are thought to be unpersuasive and not worth attention. To an extent, Mike's post bears this out -- he engages only the Two-Source Theory and the Griesbach Theory. I think that this is a shame given the strong case that can be made for the Farrer Theory, engagement with which can make the Synoptic Problem interesting again.
This all rings true for me. I first became interested in the Synoptic Problem when I had the chance to spend time coloring an old synopsis and saw for myself - though I had already been told by teachers and friends - that the data did not fit as neatly into the schema of the Two-Source theory as I had supposed.
Has anyone else had this experience?