One part of it is folks feeling like Metroid is big enough (or treated with enough reverence) that it can stand to gain a new character. As most other villanous figures in the series are either gargantuan alien beings/constructs or humanoid but mostly unremarkable or indistinguishable from the previously established in Smash Samus, everyone's settled on Rids because he comes within a certain size (hurrhurr), or more appropriately, a more iconic and malleable form versus every other candidate.
Another reason is the idea that the Smash roster is woefully lacking in iconic Nintendo villains despite having three previous rosters to work in some more. Ridley represents the face of the most recurring villain forces in the Metroid series and as such does another service in showing up in the cast.
The final reason is simply the idea that people just want something crazy and overwhelming. People want retooled Ganondorf still after all. You're also not doing Sakurai any justice stating that because of how overwhelming a force Ridley is traditionally, he wouldn't be able to make something that works in a Smash type mold.
FWIW, I'm really not as invested in Ridley as a lot of other folks are, but I support the idea because I believe that Sakurai would do the idea justice if ever inspired to and also to have another wholly new style of character to play in game. Metroid getting another representative is also icing on the cake.