It isn't what you think.
https://www.youtube.com/watch?v=OoJsPvmFixU
The problem is the logistics. The sheer number of launches needed, the time windows, the timelines with no technical planning or milestones, the inane over complexity... This was a launch vehicle and mission designed by committee to line the pockets of donors and provide jobs to pad the job markets in influential congresspersons' districts.
We're using technology from the 70s on a modern launch vehicle, not because it's cheaper (it's much more expensive) or because it's better (it's measurably much worse than modern off the shelf solutions), but because of money politics.