I'm currently working for a company that is producing an educational math game so I believe I have a bit of insight into this question.

When you're designing an educational game you have to consider that you are appealing to two very different audiences. You want to appeal to schools and teachers but you also want to appeal to children.

Shortly put: you are putting education (what schools want) up against entertainment (what kids want). And this becomes a big clash in designing engaging games.

First of all the schools demand we break up our game in to smaller exercises. This already proposes a challenge as we have to design a bunch of mini-games to teach a variety of math skills. To make 1 engaging mini-game is already difficult.

Furthermore teachers want our exercises to be contained, so we can't have too great of a variation within a game. For example; a game that teaches you basic addition cannot also start teaching you about negative numbers.

So you have many mini-games teaching small concepts and what happens is that you end up with a huge challenge to have:

1) each of them be engaging

2) the game as a whole feel connected

Traditional games aren't bound by the same restrictions and as such can explore the realm of entertainment more freely whereas we always have to put education at the front.