why does Prof Porter believe what has driven our success is starting to erode?

There are probably several reasons. Two reasons that are part of the sme problem stand out.

1) As Prof Porter notes the American political system has evolved with piecemeal reactions to current events. The means that the effect of a decision, or the cumulative effect of several decisions from various parts of our economy or political system are not examined for their effect on the whole system. The effects on the whole could be bad, good, negligible.

2) Decisions made for the purpose of changing the economic system, such a tariffs, anti-trust laws, oversight provisions etc . frequently take into account the immediate industry or situation rather than considering the overall and long run situation.

This is why the US needs an economic strategy.