Ask HN: Why can't we vote from our phones?

Here's an open question to the HN community, a technical challenge to a massive political problem:

Disregarding all political/bureaucratic reasons, what are the actual, technical challenges that prevent us* from voting online in public elections? Secure authentication? An active dB of citizens? Fraud prevention?

With the state of technology, it's time to turn tech in the public sector into a public good. Ensuring the basis of democracy (access to voting) seems like a low-hanging fruit. Why aren't we doing this? What will a secure online-voting system look like?

*Assuming USA, but could be applied to any democracy.