One reason - because they are so damn NOT fun at all!
Edutainment developers are focusing TOO MUCH on CONTENT and TOO LITTLE on the FUN FACTOR, that's why they suck so much and nobody wants to play them! But well, educators are a stubborn bunch, they insist that whatever that teaches should teach to the maximum extent, even if it means sacrificing fun (which they believe to be a good trade-off because they presume fun will hinder teaching to some level because when people have too much fun they tend to forget about what they've learned in the process).
My take - If you're afraid of people missing the educational content because of the fun they have in playing a game, then don't mix them in the first place since games are sopposed to be mostly about fun!

They want educational content in games to be as prominent as humanly possible but one thing they've failed to notice is that educational content in edutainment games should be hidden and passed on in a masked form for one simple reason - a mojority of young people stereotype education to be the polar opposite of fun, so the moment they know it's gonna be something educational and they want something fun, they get tuned off by the nature of the game. But wtf, nobody in the education sector believes that, they think it's bullshit and all from the mouth of a stupid little kid.

The most important thing is, a game should be FUN. Even if it is lacking alittle in content, it's still a successful game if it's fun because people will play it. On the contrary, if it's got like loads of content but ain't fun at all, nobody will give a shit about playing it - it's a crappy excuse of a game in the end!
Drat, im absolutely pissed. It's probably gonna take another decade or two for educators to realize that the direction at which edutainment is heading is wrong.

To summarize it all - Edutainment stinks because it's all education and little to no entertainment, and so they appear to be really lame when being digested as games. Please, no edutainment unless fun is injected inside, otherwise they're plain embarrassing. If it is not fun, it is not a game no matter how much it passes off as one!