та так то в принципе по барабану, но просто интересно, как так получилось то
The tale starts with 3DS Max. Released in 1990, it's designed and used primarily for architectural purposes. In Architecture, you typically start with your floor plan which would be plotted on a 2D grid first, X and Y. When it comes to extruding that up into a 3D model, you need the next dimension (Z) to be up. That fellow devs, is apparently why Z is up in 3DS Max. Makes perfect sense for it to be that way.
In 1998 Maya is released primarily as an animation package. Animation at the time was typically in 2D, from the side-on. Think early Disney and other old animated titles done from a side-on perspective (like a 2D platformer). You start working in X and Y, and your new dimension Z becomes the depth. That would also explain why Maya is Y-Up.
3DS Max was the modelling package of choice for most developers at the time, and as Unreal came out before Maya (and it's crazy ideas) - Unreal was done in Z-Up to match.