I can't say any of this for certain, and I have no qualifications to back it up, but I would assume that any or all of the following may apply:

1) Cost - if a you only visit an area with a risk of malaria for two weeks every year (which is far more often than most holiday makers), then taking a short course of pills that cost pennies a dose will be cheaper than receiving a vaccine that requires three doses and a booster.

2) Malaria strains - if you've been to multiple places in the world with malaria risk, you may have been given different medication. The first time I went to Cambodia/Vietnam I was given Larium, but the most recent time I took Doxycycline because the NHS no longer recommends Larium for the region. For Iran they recommended Cloroquine, which was significantly cheaper. This vaccine may only fight one strain, leaving you to have to take pills for other destinations anyway.

3) Effectiveness in adults - The article suggests that the vaccine was most effective in children within a given age range, so it may be the case that it simple doesn't work on adults. I have no idea if this is likely or even possible, but it certainly seems plausible to me.

I don't know anything about fighting malaria, but it looks to me like they are giving these vaccines to children to get them through the time when they are most vulnerable, rather than giving them lifelong protection.

I don't think (1) really applies. It sounds like the vaccine is very cheap ($20) and you simply don't take the boosters if you're not returning. At that price, the significant side-effects of most antimalarials are more important considerations.

I think you make a valid point, although I was able to acquire a course of Chloroquine to cover me for a couple of months at one week's notice, for <£10, whereas the vaccine would need two months for the three initial doses. So maybe it's a practicality argument that I'm making, rather than a financial one.

Travelers aren't at high risk for malaria. They visit cities and areas where malaria isn't usually endemic.

Eradication of malaria has been a failure because we can't reach the more remote areas where humans live with it. Because there is an animal host (mosquitoes), treating a village with anti-malarials is often a temporary reprieve. There will always be mosquitos and recurrent infections to re-infect the locals after the doctors are gone.

Is this true of, say, Chinese mining engineers?

If you are in a rural area of southern china, drink your gin and tonics daily.

If you are in a city, particularly a northern city, I wouldn't worry about it.

I mean foreign mining engineers in Africa