The main drive by Google is to provide its users with relevant information. At grammar school you learn to place a capital letter at every new paragraph or after each full stop. The main reason for this is to make it easier to read. Capital letters catches the attention of the eyes and thus making it easier to read.
Using capital letters in your Ad text has proven to be much more appealing to the eye. This will attract the searcher’s attention to your Ad text and increase the possibility of a click though. This will in turn help you to increase your CTR. If you just have a look at some of the competitive words you will notice that they start with a capital letter. This is a tip worth implementing.