Why doesn't watermelon heat well?

Crumbles are excellent for using up a variety of fruit. From bananas to stone fruit, berries to apples. And yet, watermelon seems like a terrible idea to heat up as part of a crumble.

Is it purely down the water content, or does something happen to the melon itself?

Not like it hasn't been done, but not the best structure for this type of dessert. It does come down to water content and structure...and personal preference.