Why was Nietzsche's last book called "Ecce Homo"?

Ecce Homo means “behold the man” and the book is a kind of autobiography so it makes sense though it sounds a bit pretentious.

The full title of the book is Ecce homo: Wie man wird, was man ist, which translates to Behold the Man: How One Becomes What One Is (note that it is “how,” not “why”). As you mention, “ecce homo” is what Pontius Pilate reportedly said when presenting Jesus to the crowd before his crucifixion. It is important to keep in mind that he said this after declaring Jesus to be innocent.

The other bit of context to keep in mind is that the book shares many similarities to Plato’s Apologia Sokratous, which translates to “defense of Socrates” (and is commonly referred to as the Apology). On the surface, Plato’s Apology is a presentation of Socrates’ trial and how he defended himself against the charges of impiety and corrupting the youth. But underneath that, it is also a defense of philosophy itself.

Nietzsche is doing a similar thing. While the book is an autobiography on the surface, it is also a defense of philosophy and the philosopher. In his view, the intellectual conversation was being dominated by modern day sophists and Pharisees who were scourging philosophy and turning the masses away from it. And a major part of Nietzsche’s philosophical project was to expose this betrayal and to declare philosophy innocent.

The main purpose of Ecce Homo is to contextualize his work and to battle against those who would misinterpret him or misappropriate his ideas (spoiler alert: it didn’t work). The chapter titles are overtly ironic—a nod to Socrates, who was also a master of irony. But the book title itself is also somewhat ironic. The work that comes immediately before Ecce Homo is Der Antichrist (aka The Antichrist). So while Nietzsche is presenting the philosopher to the crowd and declaring him innocent, it is a much different figure than the one defended by Pontius Pilate—a new man for a new era.