https://www.vox.com/policy-and-politics/21535103/when-will-we-get-election-results-calls-networks https://www.vox.com/policy-and-politics/21535103/when-will-w..

https://www.vox.com/policy-and-politics/21535103/when-will-we-get-election-results-calls-networks https://www.vox.com/policy-and-politics/21535103/when-will-w... tl;dr is that many of the remaining votes are in parts of the state that are more liberal than the parts of the state that have already been counted. If Biden wins a county that is 50/50 D/R, then they're assuming that he will also win a county that is 60/40 D/R (oversimplifying a bit because turnout can very from county to county and mail-in ballots might have different demographics than in person voters). However it was a bit controversial that Fox called AZ as early as they did, I don't think any of the other TV channels have followed suit