From Destructoid:

“Guitar Hero 5 downlodable content will be compatible with future Guitar Hero and Band Hero games,” a rep told us, “however the advancements and innovations made to the franchise do not allow for downloadable content to be backwards compatible with Guitar Hero World Tour.”

Those advancements and innovations include a harder drumming mode and a tweak to how vocals get powered up, but by not allowing new DLC to be played on older games, Activision risks lower DLC sales and, more importantly, the ire of the gaming community that in large part has made Guitar Hero a success.

From the same page in the comments section:

Well f**k you too Activision.