Not being in Nashville, and having no dog in that fight, I'm curious what you mean by
it's a weird thing for sure. I have not found any one reason why but I have a few theories. For context I moved here in September '21, I've been lucky to be playing semi regularly on the road with some artists (mostly covers with some originals). I've had coffee with a few bassists in bigger bands and have picked their brains about this.
so some touring bassists don't care and some do. Also in the same way some artists don't care and some do. For example Taylor Swifts bassist plays 5s when needed, and Florida Georgia Line only use 5 strings. Tully Kennedy in Aldens band is pretty much all 5s.
on the flip side the bassist for Little Big Town (he's on talkbass) tunes his 4 string basses down. I've heard down a whole step (DGCF) and a step and a half (C#F#BE). I think also maybe at some point BEAD. Im told the bassist in Brothers Osborne tunes a main bass BEAD.
also most of the "top" session guys are known to tune down vs play a 5. Mark Hill and Jimmie Sloas are the two I think of. There's one interview online of Sloas that implys he may not have a bass tuned standard.
why is this? Look and sound.
a 4 string bass looks more traditional and this is something people consider. People listen with their eyes and artists want to see what they are used to. Sounds stupid with a 5 string to some, but imagine watching little big town live and seeing the bass player play a 6 string single cut Ibanez BTB (something I've actually seen here).
Then there is sound. This is just a fact, tuning down and playing using open strings sounds different than a 5 string. So if the record is played on a down tuned bass in order to get that sound live that's the way to do it. Now I'll make it clear I don't mean better, but it's not the same and I've focused in on that a little because I want to be good at recreating the sound on the record. An example is playing the open G string vs playing fret 10 on the A string. The A string will give you a fatter sound. I hear a difference between an open drop D string and a closed 3rd fret on the B. Again, different not better or worse.
For example I LOVE Tully Kennedy's playing and sound. Like god damn one of my personal tone and playing hero's. No one gives a poop if he plays his sadowskys or some weird 7 string if he's playing the same parts. But for two different sound examples, I hear the song She's Country and the lowest note is a C# (2nd fret on the B string). That same note is played on the little big town song "Kiss Goodbye" and I am 90% sure that is a passive bass tuned down to C# standard. I personally hear a difference (not better or worse)
I've heard and I'm sure there is a bias in the studio as well, but there is also a mindset that if you sound good and play good parts no one cares which I agree with.
Someone who was a "stay away from 5s" guy made the comparison if a guitarist showed up to a country gig with a 7 string gutiar, wouldn't you have some bias? I have seen a lot of players on 5s who sit on the B all night and it doesn't sound great. Again the bass doesn't possess you to over play, but that's a bias some people have.
now again I'm 8 months in, and semi obsess learning about these things after moving here. Tag any of the Nashville pros in this thread and they'll give you different answers but I hope that explains it a little.