In general I think folks soured a bit on Borderlands 3 passed the initial honeymoon period compared to the first two.
Steam rating on BD3 is 85% and BD2 is 95%, that's a fairly large gap.
But most importantly, for Steam sales data, we have to recall that Borderlands 3 was a limited exclusive title with Epic Games Store back in 2019 for 6 months(Steam came out March 2020 compared to September 2019 for EGS+Consoles)
We even actually have the amount Epic paid Gearbox for
it via the Epic vs Apple lawsuit:
In total 146M paid to 2K, lmaooo.
Like that was legit probably the entirety of the game dev budget paid for by the Epic deal.
It also shows that 1.5M copies was sold of the game in 2 weeks on just EGS, which is actually pretty huge considering a lot of PC gamers don't give a fuck about EGS.
But yeah sales legs for the game is not that strong.