I'd wager it's because they know there will be some folks who only sub for Classic, or people will stay subbed longer because they'll have both Retail and Classic on the same account and will play one or the other.
Either way, it means they've looked at the numbers and they have accepted the cost it will take to maintain and feel like they'll be able to do so without changing the sub.
This is a huge multi million dollar corporation, they didn't just pull this decision out of thin air.