One major reason is transparency. If a company wanted to use Blockchain as a database, it would be public knowledge.
Another is it's irrefutability. If I was using some blockchain tech to write this answer, I don't know if I would be able to alter original documents. This can be both good and bad.
Most importantly, the UI must meet the needs of our basic cloud users. Without this, Blockchains will never replace traditional databases.
But in time, development of private side-chains will likely make Blockchain databases more practical and common. I really hope to see this within a few years!