Why can't dopamine cross the blood brain barrier but l-dopa can?
Both L-Dopa and Dopamine are polar molecules and thus hydrophilic. They are solvable in water but not in lipids, so they are not able to cross the Blood-brain barrier on their own. 

The difference is that L-Dopa has a transport protein (SLC7A5) that allows it to pass through the Blood-brain barrier, whereas dopamine doesn't.