Why was ROAS selected as the main KPI for success when many brands are shifting to LTV as a stronger metric for media performance?

I would beg to differ that LTV is a stronger metric. It is certainly used, but it is a metric that has many assumptions built into it that are all contestable. ROAS is objective, verifiable and directly tied to return on advertising investment…it is the extra sales you got for the extra ad dollars you spent.