They changed it because pieces were becoming fairly common and it was almost going to be worth it to convert a map. They're TNT, nothing in their game is actually allowed to have a purpose.
I wouldn't go that far. They're just trying to make it to reward people good at games, rather than people who get lucky (which is a shame, as I fall into the latter category...) Stil, it coulda been done in other ways... -sighs-
I highly doubt that motive, or else they'd still allow a person to get two pieces in an hour.
i dont know if this was posted before, and i'm sorry if i'm being rude, but can anyone tell me why i cant get 2 pieces anymore. i'm supposed to get them for for being in the top rankings and high score but they only reward me the high score one. this is getting really irritating and is wasting my time. can anyone help?
They changed that about a week ago I think. Like with all the changes, there hasn't been an official announcement or explanation.

Erm, it looks to me like all the changes are in bright red on the world challenge game page. The ability to win 2 pieces in an hour, one piece from the high score and one piece from placing, was never officially declared on the world challenge game page. That change to 1 piece an hour regardless is/was mentioned in the FAQ.