Why it was announced specifically on a sticker (rather than on the box like the other games) is the core of the question - the headline no longer really works without it. I'd go with one of:

> Why did Microsoft Entertainment Pack have a sticker announcing it had Tetris?

> Why did Microsoft Entertainment Pack announce that it had Tetris on a sticker?

> Why the Microsoft Entertainment Pack had a sticker announcing it had Tetris

But doesn't really matter that much.