Weight! and the fact they are completely unnecessary on a road bike
I remember an article about a company who had developed a hydraulic system for road bikes. The hydraulic fluid reservoir was actually built into the stem. The cost was phenomenal, and it just seemed to me that they had over engineered a solution, (superbly I should add), to a problem that simply didn't exist.
A thought I'm confident has been vindicated by the fact that no-one has adopted it.
A quick Google reveals, the company is
HERE, and the original article I saw is
HERE