It is not that hard to rotate...
Also differant branches of physics and engineering have their own
conventions... In my structures classes we always did our analysis
on the z-y plane and x was either up or down...
The vast majority of the worlds will be made by businesspeople, librarians,
individuals, etc... people who want to put up a big 3 dimensional
"I'm Here".. and these people are going to want an x y horizon and
a z height... because we tend to think of spaces as a map view with
elevation as a third consideration, as a third coordinate
And when I map out a space I tend to think 3 feet right 2 feet forward
and 4 feet up.. so I'm going to want to right the coordinate in as
3,2,4 so I think it will be easier for people to enter in the coordinates
wiht Z as up... however any convention can be adapted and overcame
just by rotating or forcing and enterance location and orientation...