Why so common?

[kilted redleg]

I wonder if RS being used as an anti-establishment way by the punk movement in the 70s played a role as well.

Kilted, this statement is one of the "legend" that I referred to in my post that Peter was kind enough to answer. By the 1970's KGVI was long dead having passed in 1952 [this year will mark 72nd anniversary of his passing on February 6th]. "Punk rockers" would often take a government symbol to use to show their distain for authority. Here's the "legend" I heard regarding KGVI and the RS tartan. The good king wanted to restrict the use of the tartan, but he had an "advisor" who was a childhood friend and retired colonel of the highland regiments. Now KGVI's real first name was Albert and his family nick name was "Berti" and while at Balmoral with this "advisor" told him of the plan to restrict the RS tartan and asked for his thoughts. The "advisor" replied " your majesty you can't restrict the RS tartan. I started as a boy piper and worked my way up to where I am today. The RS tartan is the only tartan that I know for pipers and if you restrict it what does that say about all the pipers who bravely wore into battle for the crown and paid the ultimate price, or the pipers who left service and went on to establish pipe bands and used the only tartan they knew to honor the memory of their regiments; much less old pipers like me who when they left were given a regimental kilt in that tartan and a military sporran as a "thank you for your service" by their mates. Are we supposed round up all these pipers who wear this tartan, it would be the death of piping for sure. Berti you want to restrict a tartan then restrict the Balmoral tartan, no one wears it but you royals." The good king thought about it a while and decided to restrict the Balmoral tartan instead.
That legend was told to me by a Scottish father of a friend who served in the HLI during WW 2.
My personal opinion is that "punk rockers" were a day late and a buck short when comes to the RS tartan.