The first time reaching out to a new prospect is exciting. The second time is a little less exciting. By the time you get to your third, fourth or fifth follow-up on the same lead, almost all of the novelty of the opportunity has faded...which is why many sales reps never make it that far.
It's an understandable instinct. No one wants to expend energy working on a lead that doesn't seem to be leading into anything. But studies have shown that the optimal number of follow-up attempts to convert a lead is actually six.
By giving up too early, reps pass up a valuable opportunity to win new business.