It does seem to be the norm for interesting things to be presented in a boring way, rather than the exception. I'd guess this is partly because it's difficult to explain concepts clearly in the first place, let alone make the explanation engaging...

Hypothesis: If writing educational material is like throwing a dart, you have a bigger target if you aim for just "clear" — if you aim for "clear ∩ engaging" you have less chance of being "clear".

    size(clear ∩ engaging) < size(clear) 

I think time pressure would come into play with this. If you have the time, you can hone in on the "clear ∩ engaging" zone; but if you are pressed for time you just aim for "clear" which you'll hit more quickly.

And if you're really pressed for time (and/or uncaring), you might aim for "passable" rather than "clear"; something that your colleagues would OK, but that isn't particularly great. This is a larger target than "clear".

    size(passable) > size(clear)