Because now Google, Microsoft and Amazon offer it at a cheaper price and better integrated with their own products.

The broader answer here is "Dropbox took a bunch of VC money, so 'just doing well enough' isn't an option for them and they have to stop at nothing to grow faster"

For me, the key part of Dropbox isn't first-party support, it's that a bajillion third-party apps have cross-platform support for it where availability of other services is rather more piecemeal.

And in a half-assed way syncing.

Notes-taking apps have the competition of <div contenteditable="true"/>. There isn't much left to do for a notes-taking app: Everything else is bells and whistles.

Responsibilities should be called here. Dropbox, Evernote, Slack and GitHub are all 4 hosting a massive free service, highly replaceable with cheap alternatives, with low switching costs. I sincerely hope the VCs who invest in those companies have another plan than "Add more features forever".

But the private repos and the on-site offering - the things people pay for? Fair bit of competition there.

Open Source projects existed a long time before GitHub, and will exist a long time after the cool kids abandoned it for whatever reason.

There was another article recently that said that files are old news and people are using Slack. Granted they are shipping files in that, but people have probably moved on to web hosted versions of documents anyway.