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.

yes it`s a pity they are not sold anymore. Was a nice entry product

Same for discontinued IKEA symfonisk.

My son wanted to start with sonos but when he saw the prices he abandonned

There’s a sale on currently, so you should get an Era 100 for less than the One SL was priced. 

Yes a saw this, symfonisk was lots cheaper though. And can´t really recommend him Sonos, the app improved but is still very sluggish. And missing one of my former fav functions: searching for podcasts