Well, one reason is: It's none of my business.
I was thinking when people start sharing their business with us like their illnesses, disease and weight issues and we have the opportunity to share.
At that point, I might
say, "It is quite possible that diet can have a big impact on that." And leave it alone unless they ask... I completely agree, it is none of my business. They have no way of knowing that I might know something though, hence the "hint" if it seems appropriate. But I also check my spider sense which is pretty sensitive and don't even say that much a lot of the time. Just because someone is complaining about how they feel, does not mean they are asking for my advice on how to "fix it". Those complaints and in return the sympathetic over involvement many give back, are often the coin of human interaction in our culture. And I really don't enjoy that game. But truly, most people just do not want to hear about it. They feel it is none of my/your business.
One good question to ask yourself first before mentioning it at all is, "Why do I want to share this?" Of course the first answer likely to pop up is, "I want to help." But dig deeper. Why do you want to help? Is it genuine, compassion, or is it at least in part, a desire to justify what you yourself are doing by getting others to agree with you? I think the latter is all too often in there somewhere and that is often, at least in part, where the strong negative responses come from. If there is only a clean, clear desire to be of service, then I think it often is the case that you will see that bringing diet change into the conversation is not really appropriate.