The question claims that "people are saying that X-windows is not secure". I see no evidence for this claim. I'm not familiar with anyone who is saying that. Wikipedia doesn't say that X-windows is insecure. I say it is BS.
(I know you posted some links in the comments on the question, but none of them say "X-windows is insecure". You have mischaracterized what they say. Please read more carefully in the future, and don't put words in people's mouth.)
Of course, if you want to be nitpicky, I realize you can probably find someone, somewhere, who is saying that X-windows is insecure. Heck, you can pick pretty much any statement you want, and I bet you can find someone, somewhere, who is saying that. But who cares? Even if someone is saying it, why should I care? I don't care what people are saying. People say dumb or uninformed stuff all the time. You shouldn't care about that stuff, either. You should care about the technical facts.
So don't ask about "why people are saying blah". Instead, ask about the facts. For instance, a much better question would be "Is X-windows a security threat?". Or maybe you ask a narrower question. The point is, don't waste people's time. If you want to get good answers, start by asking a good question.