It's more an issue of capital investement. The number of people trying to play the game right at launch is a lot higher than the number of people who will be playing after a month. They purchase servers based on the number of people they expect to have playing it consistently, because they don't think it's worth buying a ton of extra server capacity just to handle the high traffic they might have for the first week. All that extra server space will then go to waste.
Unfortunately, that strategy doesn't work as well these days because the poor publicity from the lack of server capacity might hurt more than cost of more servers. But it's what they've done anyways.
The other issue is that with digital sales, it's harder to predict/control how many people are going to purchase a game than with sales in retail stores. This isn't a new problem, but it's one they're clearly still struggling with. So when you combine they're reluctance to overspend on servers with potentially higher than expected sales, you get overwhelmed servers.
Which wouldn't be a problem if you hadn't made a retarded design decision to require people connect to your servers to play the game, even if they don't want to play with other people.