Timothy Gowers wrote Is the Tricki dead? back in 2010 [1]. In it he effectively said that Mathoverflow emerged after he started it, and it seems to be better at some of the types of things the Tricki was meant for. He also talks about the fact that articles on the Tricki are harder to write, and involve more effort.

[1] https://gowers.wordpress.com/2010/09/24/is-the-tricki-dead/