Well, it's who the Democratic Party primary voters of this district picked to represent them.
Personally, I'm in favor of more control for parties and vetting of who should be allowed to run for them. But the two main parties either don't believe in this or their voters would throw a fit. The two parties as they presently exist are incredibly weak entities that function as little more than ballot lines.
I don't really see the problem with the parties functioning as ballot lines. If we can't have third parties, we need to give voters choice in who the major parties nominate, or else the party bosses end up deciding everything. I see your yellow avatar so I know you're in favor of third parties, but we'd need to change the system to encourage third parties first before we limit participation in the nominating process.