Whats the differences between trustadvisor and other reviews services/sites?

I noticed a lot of people selling trustpilot reviews and also a lot of people a looking to buy these reviews, but when I trying to check some business reviews I am getting tripadvisor reviews, especially in food/restaurant niche. Also not many business are registered in trustpilot website. Is it some new stuff and slowly business starting to use trustpilot to receive reviews or what?
I mean why so big demands for these reviews when not so many business are using trustadvisor.

So whats the deal here?