No reason at all, you can in fact connect all axis switches in series if they are NC and just use one input. Mach will move one axis at a time when homing and when the switch is seen will move the next axis etc.
There can be advantahes to using separate home and limits such as if you have a very large machine you may wish to have the home switches nearer the normal work area instead of having to travel to axis extents when homing. For most however the single switch setup works great.
Hood