Remember that while Steve Jobs infamously called Dropbox a feature, not a product, it took quite some time before Apple's cloud offering was really production ready.

The same can be said for the other software giants.

Also, for Dropbox to get to $1B in revenue they have hundreds of millions of users. This is a tremendous amount of adoption. Dropbox's real benefit lay in that it integrated directly into your OS so that it appeared just like a regular directory. This meant that usage patterns for users didn't change and so adoption ramped quicker.

Google on the other hand was always more of a work suite or productivity suite.

So ultimately what happened is that as the other large companies played catch up there simply wasn't enough of an advantage to switch to them away from Dropbox.