There are multiple reasons.  First is that the speakers are essentially speakers that have to be well protected outdoors.  Even the Roam and Move, while they can be used outdoors, are not designed to be left outdoors.  Not saying that it’s not possible, but it certainly adds costs.

Second, stationary Sonos speakers require power and a WiFi connection.  While you could use an outdoor power cord, you probably want to have a better solution for something more permanent. As well, a lot of people don’t have a good internet connection outside their home.

Third, even if Sonos could produce a permanent outdoor active speaker for a reasonable cost, is there much of a market for that?  Would people rather pay more for a permanent speaker than a portable speaker they can move around if they need/want to?  Or would they rather put a cheaper, easier to wire, passive speaker in their backyard, that will be much cheaper to replace if there are issues.

One of the reasons that Sonos speakers need to be well protected outdoors is that inside each one is a computer as well as the amplifier for the speaker. And the electronics associated just aren’t normally designed to be left outdoors. Which is why the Sonance speakers connect to a Sonos Amp, which (in general), should be indoors.