Bandwidth, mostly. Most TVs, unless they are very new, don’t support eARC. It’s a fairly new system, and the majority of manufacturers are in the process of adding the appropriate electronics in their TV sets, but it’s not something, generally speaking, that could be a software update to a TV. Sonos is being proactive in including this, and making a slight bet on the future inclusion in new TVs. Wise, in my opinion, but the great majority of current TVs out in the ‘wild’ with any version of ARC will be the lower bandwidth version known as HDMI ARC. 

Here’s an article that might be worth reading

https://www.digitaltrends.com/home-theater/hdmi-arc-explained-works-care/

It takes them a while to get to eARC, but they do. 

Thanks for your quick response, however should I invest I the new sound bar if I don’t have eARC and would I not be getting all of the functionality?

thanks

The only Atmos sources that take advantage of eARC at this time are 4K Blue-Ray and Apple TV.  All other streaming hardware/content with Atmos is DD+ and able to be sent over ARC.  Also, in the case of 4K Blue-Ray, it will downmix to DD+.  So unless you are dead set on using Apple TV, an ARC equipped TV will give you Atmos on the Sonos Arc.

Yes. No. It all depends. I suspect you’d be hard pressed to hear the difference between eARC and HDMI ARC on the Sonos Arc, but seeing as they haven’t shipped any, that’s a guess. The Dolby Atmos carried through HDMI ARC should be fine, assuming you have a source that sends it through to your TV and your TV can pass it to the Sonos Arc. But even if you only have a normal Dolby Digital signal, and not a Dolby Atmos signal, I’m willing to bet the Sonos Arc will sound as amazing as my older PLAYBAR does.