The reason in a nut shell is that investor confidence in RSG is far better than WM, in terms of a stock's future earnings.
A higher P/E ratio reflects that investors are expecting higher earnings growth potential in the future compared to companies with a lower P/E.
The P/E is sometimes referred to as the "multiple", because it shows how much investors are willing to pay per dollar of earnings. If a company were currently trading at a multiple (P/E) of 20, the interpretation is that an investor is willing to pay $20 for $1 of current earnings.
So, investors aren't as optimistic on WM (compared to RSG) going forward. Both ratio's are healthy, though. It's no secret that I am an avid RSG fan!