In my philosophical overview book its said that Xenophanes was the first to criticize the greek polytheism. He concluded that there can only be one god. My book also says that this god is omnipresent and identical with the world as a whole. Therefore his "theory" is a pantheistic one.
Why didn't he "build" a religion based on this?
My book says nothing about it. Did he?
By "my book" you mean a book you wrote or a book you are reading? :)
In any case, the book is wrong regarding Xenophanes being the 1st monotheist. The first known monotheistic religion was developed by Akhenaten, centuries before Xenophanes was born. And, according to a few historians, Akhenaten's religion either evolved into Judaism (and later Christianity and Islam) or substantially influenced early development of Judaism.
Regarding Xenophanes' pantheism: he maintained a distinction between the God and the Universe; so he was kind of between deism and pantheism.
