To Noah's point, I think that Dropbox (and, for that matter GitHub) can and do devote attention to the needs of businesses, but that's not quite the same thing as focusing on capital "E" Enterprise businesses.
Just from a product perspective (putting aside the much slower Enterprise sales and upgrade cycle, which can really affect product as well), focusing on Enterprise means taking on an additional layer of product requirements that aren't particularly driven by the actual users.
The time you spend adding device-level access permission, admin auditing reports or what-have-you, to meet EnterpriseCo's requirements, is time that can't go towards the parts of the product that users actually use.