SHA1 and SHA2

Question out of curiousity here -
I notice particularly with various authentication plugins and books
like AWDWR and Ruby for Rails, SHA1 encryption is implemented. In
Rails Recipes (Chad Fowler) the authentication recipe used SHA2. My
basic understanding is SHA2 is stronger but curious as to why the
(seemingly) majority uses SHA1.

Anyone ?

TIA
Stuart

Question out of curiousity here -
I notice particularly with various authentication plugins and books
like AWDWR and Ruby for Rails, SHA1 encryption is implemented. In
Rails Recipes (Chad Fowler) the authentication recipe used SHA2. My

basic understanding is SHA2 is stronger but curious as to why the
(seemingly) majority uses SHA1.

Anyone ?

And why don’t md5?