One reason is that there's friction with purchasing. If a company already buys Active Directory and O365, it's much easier to get OneDrive added on than it is to set up a new vendor like Dropbox.

Because it can be copied and doesn't have a defendable business model.

For most people, consolidating many services into one is just more convenient, even if it means using a "good-enough" version of that feature.

I think features are often products for other companies, not for end-users.

E.g., digital displays are a "feature, not a product". End-users get value out of digital displays on their thermostats, microwaves, etc., but no end-user buys a digital display themselves, thermostat and microwave manufactures buy them.

I think the problem with Dropbox is that it's become too easy for product companies to build data syncing themselves.