Because Apple won't let you downgrade. In order to downgrade, you have to trick iTunes into doing it, and that requires files (SHSH blobs) unique to your phone that are only available if your phone was jailbroken on the old firmware you want to downgrade to.
First all, if its a AT&T phone u cant use it on verizon no matter what u do to it. t-mobile yes but no it wont work. You can downgrade the operating system but u cant downgrade the baseband. the baseband determines whether u can unlock it or not. And u cant downgrade anyways because to do that u have to have your SHSH's saved on tiny umbrella or cydia. U cant save them now, you have to save them on the version you wanted to downgrade to.