Why have schools been discontinuing the practice of giving A+ grades? My high school and undergraduate college did not offer grades of "A+." At both schools, the grading scheme was this:

93–100: A

90–92: A–

87–89: B+

83–86: B

80–82: B–

(Grades below 80 were similar for the C-range and D-range.) 

At my current graduate institution, A+ grades are given for grades of 97–100 and an "A+" appears on the transcript, but they still count as 4.0 toward the GPA, same as an A. 

To add to the intrigue, the transcripts from both my undergrad and grad institutions have a line on the back stating a historical change in policy around A+ grades: My undergrad stopped giving A+ grades in 2006, and my grad stopped counting A+ grades as 4.3 toward the GPA in 1993.

Conclusion: The practice of granting A+ grades seems to be falling out of fashion. 

But why? Did some particular event prompt this widespread change in policy, or is it due to a gradual shift in educational philosophy?
The problem with A+ grades comes when your grades are evaluated by your next institution - next level of education. In some cases at least, an A+ from one institution will be treated exactly the same as an A from another that doesn't offer the "higher" grade. But this propagates down the scale. Your A- grades are now considered B+ grades. 

Another way to look at it is that an institution that offers A+ grades may be considered to have a 4.5 top end, rather than 4.0 (I know this has happened historically), which scales every other grade accordingly. 

Aside from the scaling problem is just the hassle of having to reinterpret every student's grades.