Why was Britney's "Baby one more time" SOOO FAMOUS?

The people who say that song made pop history are reminiscing about when Britney Spears was young and innocent compare to her now. The people who review that song now appear to be excited and heartbroken at the same time. I don't get how she changed the way pop music was when there were young female pop singers before her like Debbie Gibson and Tiffany. I believe Tiffany was under 15 when she was huge.