Likely because they were once upbeat and positive people too and it didn’t get them what they want so they have changed their ways to be less positive.

So anybody who is still positive, in their experience, hasn’t learned yet that that isn’t going to get them what they want.

It might be a defense mechanism. They were taken advantage of when they were more open and positive.

It might just be bad learned behavior. The people who have what they want aren’t very positive to them.

It is also not easy to stay positive so a lot of people choose not to. Sometimes we like to make excuses for why people are able to do the harder things that we’re not doing. “Oh I would volunteer too if I had as much time as them.”. “Oh I am just not naive enough to be that positive”