Don't know the reason for sure but at the tire shops they fire employees for mounting both beads at the same time. They claim high risk for bead damage that may result in a future blowout.
I bet a studly machine could put both beads on at the same time, and I can imagine that it would put
stress on the tire...
The zip-tie method though, as I understand it, doesn't involve stress, just the opposite... it makes it
easy enough for a person to put a tire on with no machine, almost no tools at all...
-Mike "I'm trying it next time!"