I would imagine it has to do with the level of respect afforded. If I suspect that someone is just cold-emailing everyone who fits [x] query, then I'm going to treat that email with as much time as they gave it (in other words, enough time to file and ignore it). On the other hand, if it's obvious that someone spent even 5 minutes looking at my GitHub or my blog, then I can certainly give them at least 5 minutes in return (a quick "not interested" or "can you tell me some more").
