This should be straightforward enough. Currently if you receive a notification on Steam whilst in game it appears in the bottom right. This covers up the ammo count/reload timer whilst on a gun.
The two left corners would be no good either. The bottom left is the chat window, the top left is, for want of a better word, the kill feed.
No part of the UI occupies the top left, so it makes the most sensible place to put it.