They can't give us more main inventory because that IS a technical issue, it causes server load by having too many item slots available when also dealing with large scale combat situations.
That's the truth.

But that doesn't prevent us having more additional inventory space that can be turned off in combat zones, i.e. retainers.
But they've backed themselves into a corner with retainers by charging monthly for them, and they can't offer any more free retainers because of it.
Lets say X amount of players are paying for Y number of additional retainers, the moment they add a new free retainer, that changes to Y-1, which is a massive financial loss, the equivalent to reducing the sub cost by a dollar or two most likely. A loss of income in double figure percent.