Because people can reasonably predict whether they will be happy doing something before they do it. Like for example, if you polled people who had gone skydiving and asked whether they enjoyed the experience, I'd expect many would say yes. After all, they voluntarily signed up for it and paid money to do it. But if you randomly selected people and made them jump out of a plane, the results would be very different.

Also, people who have had negative experiences raising a child with Down Syndrome are presumably far less likely to be involved with non-profit organizations related to Down Syndrome.

What? People suck at predicting whether they will enjoy things, especially when they know little about them. Regret is staggeringly common. Look at divorce rates if you want a life decision of comparable magnitude. And the opposite is just as true, that people are constantly finding new things they enjoy, or even love passionately, when they least expected it.