Because the dem primary in CA would have been too crowded. Same reason Bloomberg ran as a republican his first NYC mayoral election. As a republican he could get to the general election uncontested. They’re both independent types, it was just easier to win as a republican than an actual no party Independent. And if they had run as Democrats they would have been blue dog clinton conservative democrats. (IIRC Bloomberg did run as an independent when he could run as an incumbent.)