I think, due to the possibility that the Arc has one 5Ghz channel which it uses to connect to the Sub and Surrounds, that it is tied up with that connection, and isn’t available for connecting to your router using a 5 GHz connection.
This is a guess, I’ve never tried to force my Arcs (or any of my speakers) to 5 GHz . 2.4 GHz works just fine for music transmittal around my home, and has additional range and penetration of wall properties over 5 GHz. If there’s no real benefit to me, I don’t see the purpose in forcing the issue.
On the other hand, I have forced my Apple TVs to 5 GHz, since they’re carrying a much denser video stream. The easiest way for me to do so was to rename the 5Ghz channel to something different than the 2.4Ghz channel, and just use the separate SSIDs. Since the Arcs receive their audio from the ARC/eARC connection from the TV set, there is no issue I need to worry about, and I can still group my Arcs with any other speaker/room in my system over. 2.4Ghz