1. As you said, they believe everything's already been done and they're too late to the game to garner enough attention to make the time put into their blog worthwhile.

2. They expect immediate results, and when those don't come, they quit and reassign their efforts elsewhere.

I do think, though, that if you have something to say and can reliably post at least once a week on the subject for an indefinite amount of time, readers will find you. Don't stress so much about who beat you to the punch or what would be the perfect subject to attract readers. Instead, post on what interests you, because it has to be that interest that sustains you until the readers arrive.