Let’s take the second question first. What is a wilderness? Eretz lo z’ru’ah – “an unsown land” (Jer. 2:2). What’s good about such a land is that it is ready for cultivation, ready for its potential to be realised. A wilderness has a possibly wonderful future ahead – ample reason to exult.
As to the first question – cities in the wilderness – the prophet is not talking about a modern metropolis, but about settlements in an oasis. Settlements are a promise of what can come when the whole wilderness is developed.