Why would they cast Dorothy, Scarecrow, Tin Man, and Lion like that?

I loved the way Dorthy, Scarecrow, Tin Man, and Cowardly Lion sang. Especially with those new parts of the songs and especially The Jitterbug. Even though they were awesome why would they cast Jackson Browne, Nathan Lane, and Roger Daltrey as Scarecrow, Lion, and Tin Man?