That the US is a divided country?

Sort of contradicts your original point though. But at this point we're entering Reddit-level discussion so there isn't much reason to go down that rabbit hole.

That less people show up to vote in midterm elections, those that do are usually motivated by opposition to the ruling power, and possibly that the less people allowed to vote legally is an advantage for Republicans.

Also historically the last mid terms should have given massive gains in both houses instead of the extremely slim majority in a single house.

Not really anything in a first past the post voting system where lots of people vote simply because they don't need to because their party will win the state already.