Because she topped the charts for a month with her debut song (first self-written female to do so), because she had a string of top hits, because she had several top-selling albums, because she won Brit awards, an Ivor Novello awards and even had a several Grammy nominations?
She's a musician. Unlike so many of today's "stars", she's a musician first and foremost, and not a publicity-seeking minimally-talented celebrity wannabe. She writes her own music, she draws on all sorts of both genres and influences, and is often experimental in nature. She certainly eschews both typical music industry Americanisation and commercial pressures and does her thing, her way and doesn't jump to record company whims. In other words, she puts her musical drive ahead of commercial success .... such as in having refused to do almost any tours .... and yet achieved enormous popularity and commercial success anyway.
Her work and her voice are ..... erm .... distinctive. I'd suspect most people would love it or hate it, but enough people love it to ensure her own little spot in musical history.
That's how she did it.
Personally, my question would be how the hell OK Computer came first?