Why are Showdown gold and silver rounds Live Series only

[ImDFC_PSN]

I'm geniunely curious as to why only diamond rounds in Showdown can have legends/flashbacks? There's plenty of gold cards and a handful of silver cards that could be used there, and I don't think anyone would bat an eye

[the_dragon1912_PSN]

I think just to have more diamond choices in the rounds. Take the AL west for example, there are plenty of golds and silvers to choose from but only Trout, Rendon, Verlander, Bregman, Springer and Altuve for diamonds