They also become much more attractive targets for APT since compromising say Slack is way more interesting than some avg. mid size shop.
In order to attack an on premise application that is safely behind a firewall in a private LAN, one would have to come up with more creative, staged attacks (such as DNS rebinding or blind XSS attacks).
So, put simply, whereas the attack vector of a cloud based service is basically "the internet", this isn't usually the case with on premise deployments, and therefore they are inherently safer, or at least much harder to attack.
On top of that, cloud based services are usually multi tenant environments, so the assets to gain there are huge. One serious hole in such a service (be it Slack, logz.io, Github private repos, New Relic, etc.) will probably be a disaster.
For the average mid-sized shop that's probably true. However, it's usually larger organizations that worry about storing data in the cloud.
For these organizations and attacks specifically targeted at one of them, I suppose that in order to access vital company data both social engineering and hacking an internal system are much more effective attack vectors than gaining access to a third party system.