They're probably under a contract that only allows them to publish a certain number of volumes a year or something.
And translation costs probably pale in comparison to all the other operating costs, like printing or advertising.
The time wait is worth it from their perspective if it's a better translation, I guess.
Not to mention they'll have to localise the text, making it fit the style of an English novel, rather than the Japanese style used in fan translations (dialogue on theiir own line, etc).