Why aren't wild birds eating my suet?

It may take them a while to figure out the suet. All you can do is make sure it is in a visible place near the rest of the feeders. Woodpeckers (Downy and Hairy) like it, as well as chickadees and nuthatches. Finches and ground feeding birds tend not to go for it, so maybe most of the birds you have now aren't into suet. Before too long, something will find it though.