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.
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"