Why is radio communication still used?
Cellular communication has multiple issues:


When a plane is up more than a few thousand feet, a cell phone would see, and try to talk to, multiple cell towers at once.  At best, this is inefficient and overloads the cell network.  At worst, the network can get confused and drop calls.  (not something you want when trying to get ATC directions)
When a plane is moving rapidly, it will change cell towers frequently.  This has the same problems as above.
Cell communications is designed for Person-to-Person during a single call.   Aviation radio is designed so that everyone can hear everyone else's communications.  Even if an ATC instruction is not specifically for me, I can get a mental picture of what other planes are nearby and what they are doing.  This is not so easy with cellular.
In many rural areas, cell phone coverage is limited.