Some of it has to do with making a commitment. Not so much that people are commitment phobic but that there are often other options for the week, or even that night. And YES, those damned service charges will hold people back. I know I've often waited to buy a ticket until I could get to the box office, which isn't the easiest thing for me living out of town.
With Interpol, the novelty has worn off and most of those who wanted to see them at least once have seen them. I'm sure they'll pull a good sized crowd and probably sell out but possibly not until the night of. Honestly, I'd be surprised if Travis sells out but that's mostly because of the price of the tickets. A lot of their fans will not attend solely for that reason. I'm pretty sure B&S will eventually sell out but probably much closer to the date and maybe not until the night of.
Some of the sold out shows have fans from the radio, MTV, or other means that don't interest most of the selective Indie crowd. For one thing, it seems that the Indie crowd tend to be more Euro-centric and we are in America, which would naturally be closer to the pulse of American mainstream, therefore, selling more tickets.