Newer to bitcoin can someone explain

Hello im Newer to bitcoin been listening to lots of podcasts and a few books and id like to say I understand the very basics of it. My thing is everytime I try to bring it up to people in my life they have these questions

- whats backing it / what gives it value ?

- if its so well designed why hasn't it been implemented as world currency yet ?

-why does its price go up and down so much if its got this secure well structured design?

So I would like to know whats your guy's responses to those questions so I can come up with some better answers for them

thanks