The main delay is that Mississipi allows postal votes postmarked up to election day. They then allow 10 days for these to make it through the system and then they count. By contrast, in the UK, postal votes must arrive by 10pm on polling day so are counted at the same time as paper ballots.
Then there are provisional ballots, which are votes cast in person where their eligibility is in question. This may be because the voter did not produce ID or did not appear on the register for the polling station they attended, or if their details on the database were inaccurate. Each of these ballots then needs investigation to check the validity of the vote, and may come down to litigation.
Why this is noticeable now is because usually postal votes are somewhat moot, and by the time in-person votes have been cast a clear winner can be provisionally announced. This time in Mississippi the in-person voting was so close that postal ballots or even provisional ballots will be decisive.
Pretty sure Wil Wheaton knew more of lifelong committed socialist Patrick Stewart’s thoughts on Nazis than some internet random.