I think some people are just naturally interested in politics and "the game". The game turns a lot of people off, but (obviously) cloakroom types love it. I only know a few people, personally, that get into it like I do
Then you have a lot of people that are just busy with life, try to stay informed but there is only so much time in a day. This is the vast majority of both voters and non-voters. And unless you are part of a marginalized population, for the most part the difference between republican control and democrat control is nearly nonexistent in your daily life. Though, anecdotally, trump has gotten a lot of my sporadically voting acquaintances to register for the midterms.
Lastly, the electoral college really discourages a lot of people from voting. People know which way their state will go so why bother? Yes, there are plenty of other races, but everyone knows which race is the one that gets all the attention.