Because they tend to be motivated, they (are assumed to) have congruent motives, and they tend to understand the landscape. They also tend to be cheaper, relatively speaking.

(These developers also tend to mostly work on campaign, rather than electoral, infrastructure, which makes them a poor fit here. I don't consider it a defensible decision by the DNC, but it's very defensible for most candidates to want to hire people who want to see them elected.)

Maybe for more complex projects this might be true, but any moderately competent developer could have built this.

I believe the Iowa DNC paid around ~$50K for this app. I have a hard time believing that isn't something any good dev shop would bid for given how simple the app is.

They were probably picked, to be frank, because they're the people that the folks in charge of picking a developer already had in their rolodex. Private organizations can do RFPs, but don't have to, and if you don't fully understand how bad a software project can go sideways, it's not going to be something you care too much about.

I think that's the concern people have about handing over control of anything remotely electoral in nature to them.

And really, you see this in lots of sectors where they want developers with experience in that domain even if it's not really necessary. Or at least, nowhere near as important as being a good developer is.

Everybody has a political tilt. Even somebody who doesn't vote, read the news, etc will still have opinions that are the political spectrum. Have you ever met a person (not just a developer) that doesn't have a political tilt?

Individuals make up companies/organizations so no matter what you will have a company that has somebody working on it who is not completely impartial.