Every candidate has limited resources and focuses them on the people they know will vote.
"For the rest of you, this is why the people who say they're going to "punish" politicians by not voting are just deluded. You're not punishing anyone. You just fall off our radar. We stop caring about you at all. As campaign workers, we sort voters a number of different ways, but the most important beyond partisanship is voter frequency. If you're a Democrat who voted in the last 3 major elections, we call you a "3x3" voter. We know that you're almost certainly going to show up and we know you're almost certainly going to vote for the Democrats on the ballot; all we have to do is make sure you know who all the Democrats are in the non-partisan races, which Democrats to vote for in the primaries, and which initiatives to vote for. If you voted in 2 out of the last 3 major elections, we have to spend more money on you--not to convince you how to vote necessarily, but to just make sure you get out and show up to vote the right way at the top of the ticket. If you're 1x3, we will call and harass you almost endlessly, because you're unreliable. Do you know what happens to 0x3 voters? We just don't care about you. You're not worth the time. We don't read your facebook posts or internet comments about how much you hate both parties for the NSA or Afghanistan or whatever. You're a cipher. You have far more impact on us if you're a 3x3 conservative Democrat who threatens to leave the Party and start voting for Republicans, because then we both lose your vote and our opposition picks up a vote. You would have far more impact by being a regular voter and threatening to leave and vote Green Party, but since the Greens aren't exactly going to pick up any seats, all we've done is lost your one vote. Which sucks, but not as badly as losing someone to the Republicans does."
When I ran for office, we had a tight budget. Whose door do I knock on? Who gets mail? Who do we call? You start with super-prime voters and go from there. There is no time or money to waste on people who don't vote.