Beyonce's not that great. She's good, but I don't consider her one of the "big" voices, a la Whitney, Mariah, Christina, Celine, Jennifer Hudson, etc. And her music is pretty shit. I mean...'Diva'? Are you serious?
oh brother. not big on vocals are you? maybe if you ignore everything outside the realm of what is essentially american moe-pop.
Mariah had one of the largest vocal ranges in American pop, and that can't be denied. What other female singer can/could do the things with their voices that Mariah (circa 1990-1997) could've done, and so effortlessly? Not Beyonce, I'll tell you that. Not even Mariah's manufactered clone Leona Lewis is half as good as Mariah used to be.
Mariah's voice has gotten really bad now, but during her prime, she was nearly unstoppable. Not only could she go really high (her famous whistle register), she could also hit extremely low notes. She had far more vocal range than Whitney Houstin, if slightly less power.
Nothing "happened" Her style changed over time and the songs no longer showcase her voice like the used to. I doubt you could find a female pop vocalist that can touch her now.
I used to be a big Mariah fan too, but this is crazy talk. Her voice has without a doubt worsened over time. The woman STRUGGLES every time she has to sing live these days. Her voice used to be crystal-clear and pitch perfect, but now it's airy, thin and whispery---the woman can't even sing simple songs like 'Touch My Body' live without sounding a hot mess, songs that don't even require actual singing! Saying that she's devolved into a shell of her former self doesn't even begin to describe it.