The only reason why Buffy ended with Season 7 (technically, it wasn't out and out "cancelled"; they decided to end the series before it became redundant) because Sarah Michelle Gellar was overworked and exhausted from playing Buffy for seven years. She always thought that Buffy intervene with her movie career while she was on Buffy.

She could never get roles and if she did she would have to drop out last minute to film a season of Buffy while filming said movie. Apparently, Sarah Michelle Gellar was OVER playing Buffy by the third or fourth season. She was getting older and wanted to become a movie star (Buffy ended when she was 25).

Also what did Supernatural have that Buffy didn't?

I don't know how true it is that Buffy intervene with Sarah's movie career. She had quite a few movies released during Buffy's run. I think she was creatively bored with Buffy and that's why she quit. During season 6, she couldn't stand the story on Buffy. She didn't agree with what Joss had done that year and was expressive about it. If it really was about her wanting a movie career, she would have been in more than what she was.

As for what Supernatural has that Buffy didn't, there's nothing really to compare. Buffy is a different show than Supernatural. Both are great in the genre, but they tackle the genre differently. Even though Supernatural has a longer run, Buffy is still considered the best in the genre with critics and fans. I feel what helped Supernatural had a 15 year run is that there's only two main actors. If it was an ensemble cast like Buffy, it wouldn't have lasted as long.