Chief judge of each court basically has their own say, plus whatever their admins/clerks want.
There are 94 federal judicial districts. So it's like having 94 different clients.
Nobody at the top (which is really the supreme court) is really willing to say "no, we are all going go this way", because the people at the top are judges, not technologists. There is a tremendous amount of politics involved.
All that said, i'm not defending them. THey have 164 people working in IT. That's plenty to build a functioning database system. I've seen some public judge apologists for the complexity of pacer: that nobody understands how hard this is to build, and that it's outside the realm of normal technologies.
That, to me, just demonstrates the problem: They think their problem is very very hard, and while not easy, it's not that difficult. People build systems that handle number, scope, and complexity equivalent to PACER every day.