Tl:dr: storage is expensive, and we aren't going to be giving it away when you're not giving us enough revenue
Maybe: Storage is expensive, and we'd rather collect money from you, rather than sell you to advertisers.
As an aside, if you were looking to subscribe to their Pro product, the link in the blog post contains a coupon for $15 off their yearly subscription.
Servers cost money. People making the software and running the servers cost money. Previously those costs were hidden from end-users, usually by advertising-based business models. Now that consumers aren't putting up with that as much, smart companies are doing the old-fashioned thing of charging people for the valuable services they provide, instead of selling off human attention to advertisers. On the whole, this kind of stuff is better for everyone.