For simplicity sake:
Destiny uses 10 "servers" to handle its everyday player base.
Bungie know they need 15 "servers" to handle a DLC launch, but those additional 5 servers cost (pull a number out of thin air) $500,000.
3 days after a DLC launch Bungie know 10 servers would continue be enough to handle playload due to knowing that's the time it takes for interest to wane as the immediate "must play this at the same time as everyone else" feeling is over.
Why would Bungie pay money for more servers, and more maintainence costs for what would be 5 idle servers that are only used for 3 days every few years?
Why would Bungie fork over huge amounts of cash just so people on the Internet, who have already given their money to Bungie, be satisfied with no queue times?
If you have a problem with this then don't pre-order. Don't rush. Don't be manipulated into having something right now. Have control. Wait.
No business makes money by wasting it on resources that aren't needed in the long term, only for a couple of days once every few years. The player will get over their anger, they always do.