When measuring the fall time, why should you find it between 90% and 10% of the curve?
10% to 90% has evolved as a de facto standard. It could easily have been 20%-80%, or 5%-95%. But it wasn't. 

When you make a measurement, you want it to be reproducible, which means going through the trigger point with a reasonable slope. 

When you make a common measurement, you want it to cover as many cases as possible.

Using nice round numbers like 10% and 90% has satisfied enough people on these two points that they keep using it, even though it might not be exactly right for the specific system they're using.