Possibly due to cheaper overall production costs building only one model, rather than one with a mic and another without.
This was also result in less development time as compatibility in the Sonos system overall only has to consider one model.
The physical mic switch on newer models enables/disables the microphone, so those that do not wish to use the mic can leave it disabled, but if at a later date, they require the voice capability they can easily enable the mic, which is not possible with an SL model.