Absent colossal fuckup or major scandal, it really isn't that hard to be a fairly popular president for your entire term in office. Sure some random winds can hit - like a recession - but otherwise the US population is fairly forgiving, and at least until the Trump era that was doubly true of "just give him a chance" Democrats. Obama was a consistently popular enough president for his entire term. Republicans mostly hated him, Democrats mostly liked him, and frankly that's how it should be, notwithstanding all the rhetoric about "unifying the nation" blahblahblah. People disagree about stuff, elections are a competition between visions of governance, and the idea of a super popular president is, to me, more horrifying than desirable. George W. Bush was super popular for awhile. You might remember why. Generally, one side wins, one side loses, and the losing side is unhappy. To get below 40% you have to start losing your fanbase. To get much about 55%, aside from the new in office honeymoon, there's probably just been some horrific tragedy.
With a little luck and a lot of competence (not even personal competence, just in the people you hire), you can be "as popular" as Obama, who wasn't actually super popular.
All of this is just my way of saying - if you're sub-40% for an extended period of time absent major recession or clear scandal, you really have to suck.