Well, seeing as we've not proven life can exist on a gas giant like Jupiter or Saturn, they're presuming from that fact.
We've yet to detect a planet close to Earth size (ie where gravity is 2x or less Earth's) in a "habitable" zone (that distance from the star corresponding to where Venus, Earth, and Mars sit), so therefore not habitable.
Could there be a planet that would allow that around those stars as well? Maybe. Could there be a moon of one of those gas giant planets that could, as Star Wars showed us? Again maybe. We don't yet have the ability to notice them, so we can't say.
The main reason this is significant is because until now our solar system was the only proof we really had that you can have multiple planets orbitting a star. We've detected single planets conclusively, but we've only presumed they were really "planets" and not just a failed binary star. We've never known if the presence of one such body in orbit around a star might prevent the formation of a regular solar system. There's always been this "is the solar system a unique oddity?" concern among cosmologists. The fact that we've now seen another, so close to us, indicates the probability that there are many similar others has gone up exponentially.