For most people, wireless spectrum is a topic best discussed right before bed with a warm glass of milk. It is boring. But it's important. While landline internet is, as we know, a series of tubes, wireless internet is more like a giant fleet of invisible flying trucks... or something.
To put it plainly, long-range, high-bandwidth spectrum usable with cell phones is a finite resource. Now, the scarcity of that resource in reality is very debatable - vast swaths of basically unused (or severely underutilized) wireless spectrum are in this range, much of it belonging to the military, public safety, television, and various executive agencies.