Why have schools been discontinuing the practice of giving A+ grades?
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.