Primaries elections, etc.

Why do we have this system of primaries? Specifically, why is the process spread out over such a long period of time? The result is that vast numbers of people are shut out from making their preferred choice by the arbitrary fact that they happen to live in a state that has a late primary. Is this another one of those little things built into our system to ensure that we don’t get too much democracy, like the Electoral College? I want to vote for Bernie, but by the time I have a chance, it will all have been decided.