What makes you say that?
Well ... the short statements that you quoted. To say: "Heavier-than-air flying machines are impossible", or "X-rays will prove to be a hoax", or "Radio has no future", are all dogmatic statements of purported certainty, not scepticism. To say that heavier-than-air flying machines are impossible is to say that such things are ruled out of court once and for all and for ever. A properly sceptical statement in Kelvin's time would have been: "We currently have no evidence that heavier-than-air flying machines are technically feasible, and their existence is unlikely given the current state of our knowledge and technology. However, we must reserve judgement since we don't yet know what future evidence may lead us to." Last night I watched a
Horizon documentary on time travel; though the general position is that Einstein was right and that the speed of light is an unalterable universal constant, some physicists are taking seriously the possibility of superluminal travel. There's no evidence for it and they're in a minority, but they're serious scientists, not swivel-eyed cranks.
That's long-winded but accurate. Perhaps that's why dogmatism appeals to people, fundamentally: it's quick, short, simple and easy, whereas reality is convoluted and complex. It's easier to say that flying machines are impossible than what I wrote, even though he was wrong and I'm right. Kelvin, by the way, died in 1907 so he lived long enough - by four years - to see himself shown to be wrong about h-t-a flight. See his Wikipedia page for a rather painful demonstration of how arrogantly wrong he was about how many things.
It's a shame that you picked a particularly egregious example, since Kelvin - just like Fred Hoyle, by the way - is principally remembered today in the history of ideas (rather than in physics, where the Kelvin scale is still widely used) for getting some very big and very important things completely and utterly wrong; in Kelvin's case, the age of the earth (because he didn't understand radioactivity), in Hoyle's case ... well, it's a long list.