Because Sonos hasn’t had the opportunity to add it?

You can see a list of supported services here. I would imagine TuneIn is to be added at a later point. 

​@Wolf2024 

Hmm, not sure what’s the official status on that, but I can confirm I can use SVC to start local radio stations here in Germany playing on my Sonos speakers. 
 

Edit:

Did some further testing and it seems that some TuneIn stations can be played that are available via Sonos Radio. For German stations it seems also depending on the name of the station. Some can be understood by svc and others not. For example svc has problems with station names existing just of letters and numbers like „wdr2“. 

I am betting they have no intention of adding TuneIn

Why? They have a working relationship with TuneIn for Sonos Radio and the new app has made the system more dependant on TuneIn to ad links to stations TuneIn does not do.

Because they want to sell Sonos Radio would be my guess, so they won’t develope TuneIN.

How do I know this, I removed all instance of TuneIn and then asked it to play my local station by frequency and it played. It had an ad at the start so I will monitor throughout the day!  On my phone it says The radio station but also says UnKnown Content.  Even says Sonos Radio

And if you go and set Sonos Voice for location You can have it read the weather for your location if you want to know how it will be today. 

I should also say  it uses TuneIn for this.

One thing in case it was not clear I have to say not only the frequency of the radio station but also the name.

Example - Hey Sonos, Play Big 105.5

Then I can say Hey Sonos pause the music

and Then I can say Hey Sonos unpause the music

One thing I don’t get is when I open teh Sonos app and look at Sonos radio , it does not show me local stations. It starts with trending.

The reason I am trying to figure this out is because of this article https://www.cnet.com/home/security/amazon-is-canceling-this-alexa-privacy-feature-on-march-28-should-you-worry/

I am not sure if I don’t have any alexa devices in my house , what it will do on Sonos?

I don’t see the problem at all… SVC works like expected and regarding to Alexa you will still be able to use it like before. These changes to some Echo devices supporting Alexa+ make some people reacting with panic. At the moment imo there’s no information about changes on Sonos Alexa usage. 

Just so you know I am not paniced. However I am working through the process of using Sonos Voice and not using Alexa. 

Of course… and imo svc is a really good option to control playback and choose content to play. But many people also like their voice assistant to answer questions, control smart home etc. 
So speaking for myself I use the combination of svc and Alexa and imo their is no reason to cancel Alexa.