Why is there no fixed timetable for making partner in consulting?

There isn't a strict, hard-and-fast rule for this thing. It's highly dependent on office, time period (i.e. bad times or good times), etc. etc.

I do remember a partner promotion party where a few of us were talking. One of us turned and said "Do you think they made it because they're better, or survivors?" All of unanimously responded "survivors".

So, getting promoted at MBB is as much about persistence, hardwork, determination, and drive as anything. There's an implicit table-stakes assumption that you're doing your job, but you absolutely have to "outlast" the others.