You are unlikely to get any official response from Sonos as they rarely comment on why something has or hasn't happened. 

But I think Sonos probably understand what they are doing better than you or I, and haven't forgotten anything. 

I suspect Sonos have avoided 5GHz in the past (except for HT) because of the superior wall penetration and distance capabilities of 2.4GHz. That  is  crucial for multiroom music systems (but not for washing machines).

I also suspect that more recent speakers have more powerful radios in them, giving Sonos more confidence in 5GHz for those speakers.  Older speakers have been left on 2.4GHz for reliability reasons.

As I say, that is purely my interpretation of the situation. 

If, as I recently discovered, a solid 2.4 environment is more than enough for HD video streams from Netflix and the like, why does Sonos even need more for audio?

Perhaps to avoid interference in a busy 2.4 environment, but at the cost of much reduced range.

Because speakers are not the only WiFi devices around.

The 802.11b is now 20 years old and, to put it mildly, is now obsolete. 
There are only 3 non-overlapping WiFi channels in the 2.4 GHz band therefore we can better use of them thanks to 802.11 g/n.

One could also run the system over SonosNet instead of using WiFi connection.

I just received my very first Sonos speaker yesterday (a Sonos One Gen2). Looking forward to setting up my new speaker i had issues in the wifi setup. My 5Ghz network did not show in the list, only my 2,5Ghz network. I tried the “Another network” solution where you enter the SSID and password, but could not connect.It says on the Sonos websites that the speakser supports 5Ghz networks (and that is  also the topic of this thread). Any ideas?

My network is a mesh network. I have the Asus XT8 mesh system with one router and one node. I have split the 5Ghz network and 2.4Ghz network into two separate networks for better control of IoT stuff. 
I used my iPhone Xs with the latest Sonos S2 app (the one with the brown icon color).

Any tips, ideas or advice would be nice :) 

Please don’t double post. Or hijack other threads.