For lower traffic sites, it just doesn't make sense. A site that gets 1000 uniques/day then gets 2000/day isn't going to kill an advertisers budget.
What does a lower traffic site have to do with the fact that it's concurrently fighting for ads with all sorts of sites including higher traffic site?
Just because your site isn't going to kill the budget doesn't mean your competitor isn't killing it at that same time. Everything happens globally in real time and your site has no bearing on what other sites are doing and their impact on the global AdSense advertising pool. The advertiser money may run out at any minute, regardless of your traffic, it's a worldwide global thing, and even ads showing on Google SERPs are dipping into that same ad pool at the same time.
You have to think in 3D as this isn't a 2D ad system.