Because there's a layer of air there with a different temperature or pressure (depending) that's not conducive to cloud formation. It's the same reason that very tall clouds, thunderheads for instance, have flat "anvil-shaped" tops.
in convective cloud formation, warm humid air rises until the point that it is cooled sufficiently for water to condense (i.e. reaches its dew point). This point is called the cloud base, and significant condensation will not occur below it. As long as the air has a fairly uniform temperature at a given height, and the rising air is of fairly uniform humidity, this should give a pretty flat cloud base.