'Pound' hasn't really become 'hashtag', the character # itself is still called the pound sign in American English. But in British English, it's commonly called the hash sign, so it's easier to see the evolution of the word from there.
'Hashtag' itself refers not the character, but to the character and the word or phrase that follows it. It's a portmanteau of the words 'hash' and 'tag', which accurately describe its appearance and function. It's a tag that's used for performing searches through social media (or other media that's implemented it), and it's prefixed by a hash sign.