Probably because it's a dangerous place to be in long term. Storage is going to get cheaper and cheaper from the big guys. Already for me there's no point to paying for DropBox when I can pay for iCloud for similar prices and get all the benefits of the integration that storage provides that DropBox doesn't (photos, etc) for the whole ecosystem.

I've never needed to get more storage with DB… the free was always good enough though I'm also sure I'm not a big expense to them… many free users are.